While looking for larger pop pop engine materials I noticed the possibility of using large steel can parts. Below are parts of a large steel food can. The lids are 6" across and the can is 7" tall. When placed together, the lids could make a very thin flash boiler, and the can body can be flattened to maybe do the same. Note that the lids are cut off with a side cutting "safety" can opener, which leaves the top seam with the lid instead of with the can body.