Cobbing a Micro Burner

Hi folks, hope you're all well, so I got the Rocket Mass email yesterday and got me thinking. Really, I need a bigger burner but, for now, I'd like to see what I can get out of this little smoker/burner. I was thinking of cutting tiles and creating a 2-3 inch thick wall around the outside of the burner (it's square) so that the chamber burns hotter and more heat into my caravan, plus underneath it etc. After viewing the email, I got to thinking that I could potentially cob it.

Can you cob directly onto a burner? I was thinking even the tiles might crack and hence need to be heat proof.

It's a top loader. As you can see, I've added a couple of large stones to the set up and these help no end. I would potentially keep the top stone and could weave the other into a cob surround. Flue is a flexible steel exhaust with added outer steel tubing so is twin walled, kind of. It has one of those aluminium cooking stove windbreakers which also helps to keep the burner warmer.

I'm currently saving for a van to convert to a camper and will look into doing a light weight rocket mass stove in there. I'm about to purchase a second hand tile cutter and cheap recycled tiles to do it but thought I'd ask here first. As you can see, it's sat in a cast iron cook pot- this could be filled with cob or similar potentially. I'm guessing concrete might not give a clean odour when hot but it needs to set hard so as to conduct heat.

Any thoughts on a simple, cheap/free way of increasing efficiency? Nothing too complex or costing £$