I wondered this myself. I think there's a market for a "blister" out there as well. Obviously you would have to modify it to fit your application.
1950's to 1960's Harley Davidson Primary covers; Good kitchen supply stainless steam table pans; (come in many sizes & shapes, with flanged edges) You'd be surprised at what I used for large scale model airplane cowls! (large size was 'weight-forgiving', and planes were Control Line types) Lots of 'junk' around for projects like this! Junk stores and flea markets yielded raw material galore.