DIY floor pans.

So i have ran into another problem I found large amounts of rust in my rear passenger side floor pans it travels all the way to the seat mount and up the tunnel as well. Unfortunately I cant afford to replace the whole thing and this single pan isnt gonna be big enough i dont think so I'm thinking about doing one myself. Just buy a sheet of sheetmetal and try and form and mold it to match. Here are my questions. What gauge sheet would be best? As far as making the corners is it best to cut a slit and fold it like a box? Should I try to do it in one large piece or in sections and weld the sections together? Thanks for your help and please dont reply "just buy them" because I cant afford them I have looked. Unless i can find some used that someone has cut out of a parts car. The whole pan is like 500 plus 150 shopping I just cant swing that. Here are some pics if it helps.

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