Floor under RMH

I have an out building built on grade; it had some moisture/drainage issues that are being dealt with, but in the repair I removed part of the plywood floor. I intend to build a RMH with cob bench. What would you do for the floor in this case? Pic attached where floor is removed.

Manderly,

If you are already by the grade I would:
-trim the floor joist to create the opening for the RMH slab
-add some perpendicular boards to stabilize the cut boards
-compact the area under the slab
-add base rock or gravel and sand (2-4 inches) and compact well (it can go over the existing grade, or you could remove some dirt first)
-you can add a vapor barrier, if not then I would increase gravel thickness
-make a simple form, 4-8" high, set it, level it and stake it
-add rebar reinforcement
-pour concrete

This way after two days of work, you ill have a proper slab for your RMH.

Thank you. A few more questions:

There is vapor barrier under the joists already. Should that be cut out as well so the gravel/slab directly touches earth? Or okay to just build on top of that vapor barrier? Should there be some insulative layer between the ground and the slab? Does the bench require a slab as well or just where the heft of the brick for the stove goes? In which case, would plywood with cement board on top be advisable?

I know a lot about how to build the stove/bench but this base layer inside on a wood floor confuses me a little.

Maybe I just make a big slab for the whole thing?

In theory you could pour concrete over the vappor barrier, but you do not know if the dirt under the barrier is compacted, if it gets moist and since the barrier is old, it could get damaged during slab construction and then seep moisture to the slab.
If the dirt is not compacted, entire heater can tilt or crack. It happened to Thomas here on permies.

The best would be:
- remove the barrier
-remove dirt under future slab down 4"
-compact it
-add 4" of base rock (or gravel/coarse sand) and compact it
-reinstall the barrier (if needed to cut, please make good overlaps of 12" and use proper vapor tape)
-pour slab over the barrier

I would do continuous slab under entire heater/bench. You will feel good building on it and the heater will feel good resting on it.
Please make sure that you add reinforcement, as slabs (especially longer ones) will crack if not reinforced. I would put continuous rebar #4 (0.5") on entire perimeter of the 4" slab and the same rebars in the middle to create 8x8" grid. For thicker slabs there should be two layers of rebars on top and bottom. It would require some bending, but it's worth. If you provide the shape with dimensions I can draw for you rebars with bends.
Alternatively some heavy duty remesh could be used - not the thin type from big box stores.

On the slab you can build an insulation layer using one of the techniques:
-pouring perlite/cement mix
-laying regural bricks as rowlock stretchers to create cavities to be filled with perlite or vermiculite or even broken glass
-laying insulating bricks
-laying insulating boards

Wow. Thanks so much for the good info. I’ll post here when I have a solid idea of dimensions and what I’m cutting out for the slab.

This is going to be my woodworking shop and need to be careful about leaving room to actually work.

If you are turning it into a shop, you could consider to turn entire floor into a level and solid slab. Heavy machines will like it.