So can you weld? Sorry, but I read all your responses and didn't see. If you can weld, You can definitely do this without buying floor pans. I would use 18 gauge steel, Here is a question. How pretty and original do you want it to look? A little overlapping on the panels (I would do several smaller pieces in each area) will make the job easier and much faster. The slightly rounded corners can be done with slits and made to look pretty good. There is a way to make them, but you will cut more than one slit in each corner. I am doing this in bed. If I forget to send a few pics tomorrow, don't hesitate to send me a PM to remind me. Good luck.
LOL in advance: Why not just use fiberglass like everyone else?