Some of the unplated steel I used have to make TN patchboxes was from Lowes & is .044". Very slight curve to them and seemed plenty stiff. I've have had no complaints on them. Might be a little thick if you were going to press them into a dome or something.
I tried some that was too thin once, but don't recall how thin it was by ". If it is too thin, every time you open/close it, it eventually may not stay in snug to the inlet, especially if you have to press it in snug to latch it. The thin one I made after opening & closing it dozen or so times, I noticed it was appearing to bow in.
So I laid a straight edge on it & sure enough it actually was bowing in. That got me to thinkin, if I snag it on something, or bump it with it open, I could spend a hour reshaping it or making a new one, and that involves browning a new one & etc. That is sure to pi$$ a customer off. Thus I decided if I be making one for now on, I would prefer thicker vs thinner metal,be it steel or brass.