My Austin 7 special is mostly aluminum skinned, with minimal framing.
The sides of the body around the driver/passenger are 0.7mm thick, while the top panels and those around the engine are 1.2mm.
I wouldn't use less than 1.2mm for anything you might lean on!
You can see the minimal framing in one photo, in the other the 0.7mm panel is the silver one.
The thin sheet is folded around the frame where possible, and otherwise the edges are folded back on themselves (about 25mm).