Why not try something really safe and available in many different configurations…a fuel cell.
Today’s fuel cells are available with senders for gas gauges too. Most of the companies offer “street” style cells that are not extremely expensive. You would want one that has it’s own metal “box”, not the black ABS plasitic style you see in the trunks of street rods.
ATL, JAZ, etc are some of the companies…do a google search ro any where that sells parts for race cars (Stock Car Products, etc).
Any decent welding shop can fabricate a “nest” to hang the fuel cell off the rear frame rails. Look under any circle track race car to see what I mean.
I had a CMC Pinto based MGTD and it came with an aluminum gas tank that fit between the rear frame rails. I had to fabricate a “nest” to protect it from dragging too.
This is the fuel cell in my ’85 Mustang Road Racer.