homemade expansion chamber; what gauge material?

I'm making a hydroform expansion chamber for my college project but am struggling to find anywhere a good idea for what thicnkess material I should make it out of? I have a guide of how to do it, a book on how to work out correct length and shape but no where to tell me the thickness! I noticed that a couple on here have made their own so if they had any ideas, would be a real bonus. Cheers

 

cheers, just what I needed to know