Tensioning thin sheet metal is interesting. I'd try to get wider sheets than flashing, though. If you didn't get it right would it sound like one of those thunder sound effects machines? Not sure about the expense vs. cloroplast, mostly because of my ignorance of cloroplast! The heat resistance definitely has appeal. I've never made a belly pan, either, unfortunately.
On a quasi-related note, I made my own air handler for generating hot air for heat and I used a very thin gauge skin. Unfortunately whenever the fan kicked on it would inflate like a balloon and made a noise when the metal face would pop out. I finally put a ratchet strap around it with a block of wood on that side to keep it quiet.
On an even less related note, the first American ICBM, The Atlas, to keep weight down was made with no internal structure and was essentially a huge stainless steel balloon! It varied in thickness but was around a dime's width.
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I bought a sheet of coroplast (corugated plastic) from a sign shop. they charged me $20 for an 8x4 foot sheet. it is much cheaper than sheet metal unless you have it laying around. free is always cheaper.
ended up doing nothing with it for a few months and giving it away. the new owner put it to good use though.
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