Koen would be the man to say how small the diameter can be.
Isn’t the housing wall thickness a structural issue? Or I do not understand the question.
I have been using 20 and 30 lbs propane tanks, if that helps. Gary Gilmore traded me a 16 gallon (or are they 15 gallon?) barrel. It is in such good shape that I could not bring myself to use it. But now I have a lite weight design to try and that barrel will find a home.
The best container to use is the one you already have until you have your gas legs, so to speak.