Batch rocket bell cap

Been looking for more info but couldn't find much while sifting through permies. For rocket mass heater bells using a brick bell rather than a barrel, what are people doing for bell caps?  I think I saw peter using a cast plate, but wanted to find a recipe. Any additions to the refractory cast to make it span a 2 foot distance? Any other materials being used like marble or soapstone?  Thanks for info or links.

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.

I used corbeled-in firebrick to cap my 26" x 26" inside bell. It was easy and self-supporting at every stage, once I figured out the right configuration (dry-fitting a mockup on the floor before building for real). Photos in the thread:
https://permies.com/t/60784/Rocket-mass-heater-tube-bell