Mostly theoretical advise here: add rockwool to the refractory.
I have made some rugged cores from RapidCast CementAll, perlite and rockwool fibers.
When I left the rockwool fibers out I got significantly more cracking.
Sheet metal could be used to help with bridging.
I have actually poured CementAll into the lid of a pocket rocket. I drove self tapping screws into the lid, wound some wire around the screws, and growled in my mixture. I used the bottoms of tin cans,screwed to the lid in two places, to leave space for a
feed tube and chimney.
It has held up well,though the feed and chimney have not.
If I do another such lid, I think I'll cast around the greased base of a large stainless steel stock pot.
After removing the pot, I would grease the resulting hole, line it with foil, and fill it with more refractory, with some sort of handle set in the wet cement.
Thus equipped with a pot and a plug,I could boil
water for food,distillation, thermal mass, etc.
