Brandon - what are your steel fabrication capabilities, tools, etc.? I have a pretty complete home fab shop (plasma, mig, tig, chop saw, acetylene, BMFH's etc.) and I wouldn't even consider scratch building the housing for a QA-42 unless there were absolutely none available on the market. Buy a couple now, restore 'em and set 'em aside - you'll have enough for the next 20-30 years..
If you are determined, the first thing is to find a supplier to roll the steel - you give them the dimensions, they'll give you a basic rolled piece - I'd do it in at least .125 steel..
By the way, welcome to the forum - I'm down the state a little ways from you, but hang around "Lumber Town" quite a bit these days..