Alternative or handmade materials for a durable stove firebox?

Interested in creative solutions for building up a simple custom fire box from basic materials at hand - i.e. clay, stone, cement, etc - for a (protected) outdoor wood cookstove-oven assembly. I have done some reading on here and get the sense that combustion chambers are generally either metal, or fire brick, with some specialized cement based mixes that can work as well. The gist being that clay/earthen based material does not hold up to repeated high heat and will crumble with use. Is that a correct assessment?

If I'm lacking those preferred options (without significant cost or scrounging) and want to try and improvise an alternative, is there an obvious second tier option or options? I'd like to try some clay-mortared stone and see how it holds up. I'm not opposed to doing occasional maintenance touchup, and this stove won't see heavy or intense (high heat) use. The biggest challenge would be a decent 'roof' layer, which I'd be willing to do a custom steel reinforced cement mix to keep it on the thinner side, say 3-4", maybe incorporating old clay brick on the upper face.

Still early in design process with this, and getting a sense of what's reasonable before finalizing plans, so I welcome thoughts, tips, or reference points. I know there's lots of accumulated knowledge here and maybe some good discussions that I missed in my initial search. I will elaborate on the design in a separate thread when it's ready to go, but I'm hoping to have this approx. 20"x20"x16" firebox placed underneath a typical domed earthen oven, with one flue pipe leading up into the back of it (for heating) as well as a second close-able flue going up and out the side into a single 'burner' cooktop set into a small cob countertop.

A few advices for a bread oven.
For the firebox material you can use cob - clay with sand and some straw for reinforcement.
For insulation you can use light clay (straw with clay slip) or perlite mixed with clay slip or perlite mixed with cement (1:10 ratio will provide awesome insulation).
Floor, walls and the ceiling should be the same thickness - 10cm/4" or 4.5" half firebrick will be sufficient for home use. Bakery ovens have full brick (9") thick floors, walls and ceilings
Floor, walls and the ceiling should be well insulated and damper installed to prevent quick heat escape.
Well build professional bakery oven can be heated and store enough heat to bake even 7 batches of bread.

In traditional and modern bread ovens the bread is baked in firebox after the coals were removed. Gases can heat a second chamber but to a lower temperature than the firebox. I can easily heat my main oven to 450 C, but the top oven/smoker never reaches more than 200 C, which is still fine, because breads are usually baked first for 10-15 minutes in 250 C and them 45 minutes in 200 C. The top one is good for baguettes, pastry cooking stews and soups overnight and smoking. The main oven will bake pizza in 5 minutes.

Thanks Cristobal, that info matches my sense about cob oven design and performance. In this case, I'm making the oven more as an experiment, for occasional use based on its performance. I think it will be useful for some form of cooking/baking, but I'm not too worried about getting especially high performance for any one specific use. Ideally, when firing, it would swirl the hot flue gasses and perhaps allow some secondary combustion rising out of the fire box below, as well as some conducted heat through the oven floor / top of fire box, to pick up as much heat as desired. When its up to good temperature, the fire can reduce to coals and sustain heat for a while during cooking use (either with or without continued flue draft, since it can exhaust to the side as well).

So, my main question here is how to build up the walls and roof of the firebox, on top of an existing stone seat or 'frame', so that they will hold up to repeated firing, without installing a metal box or lots of brick. I've done a good bit of burning inside another stone-clay fire box underneath my hot tub, and that's survived okay with some repairs as needed. I guess I'm curious what else could be done to improve the durability of earth and stone, if that's my default solution, incorporate cement/lime/other additives for integrity, or another option I haven't considered.

And I should have mentioned, I do intend to use a metal front face and door with draft control to enclose and regulate the fire that way.

I think cob oven that has a flat ceiling would be hard to make and not very stable.
This is one reason domed ovens are popular
Most of the tutorials I seen for making the domed ovens call for cobbing over a form, that has been constructed from damp sand.
After the cob sets , the sand is removed through the doorway of the oven.

If you don't object to metal, just cost, cast iron griddles can be had for cheap in second hand shops.
They have even been used as ceilings in the burn chambers of batch box rocket stoves, so they should hold up in this application.
Stainless steel steam table pans or even regular steel sheet pans might serve your purposes as well.
For a steel dome, try stainless steel mixing bowls.

My first oven was made of cob dome that I formed over a pile sand covered eith plastic bag. Around  10 cm thick, no insulation and non insulated floor that was sucking all heat. The opening for fuel was also the chimney. It was quite miserable and very difficult to bake anything.

My second one was a flattened batch rocket firebox - fi 200 mm equivalent, made of dry stacked red bricks. No insulation, but we managed to bake some inferior breads and sometimes good pizza.

Based on that firebox I built a real dual chamber oven, with firebricks, a lot of insulation and real cast iron doors. It burns super clean. I love it, but would make one improvement - complete insulation between the firebox and upper chamber, so after the burn, the upper bricks would not suck firebox heat. We bake a lot of breads, rolls, pizzas, make stews and smoke. Pleasure to use. It keeps heat for two days, so it can be quickly reheated to baking temperature.

You said that you are counting on having the oven floor to be heated by firebox ceiling. My measurments consider it to be inefficient. When the brick outside is 70 C - hot to touch, the interior is 400 C, so to heat the floor sufficiently, the ceiling will have to be heated to red hot temperatures and it will quickly compromise rather thermally delicate cob. That's why I always recommend traditional approach, used in all bakery and pizza ovens: firebox is the oven. This way you will always develop sufficient temperature without wasting fuel and the oven walls. With insulation all around and chimney damper it will always work. Around 40% of wood burning energy is emitted through radiation from flame and coals. When we add conduction of coals and convection heating of the firebox it will turn out that at least half of energy is in the firebox. The rest is heating the fues, other chambers, chimney and air.

If I wanted to build an oven as separate chamber I would build the firebox from insulating firebricks to direct as much heat as possible to the separate oven. I would also make the ceiling from metal (would need to be some quality heat resistant alloy to survive). I would also pad the door with insulating material to prevent the losses due to radiaton towards the door.