A lot depends on the equipment you have available to make a fuel tank. It would be fairly easy to
make one like your original using aluminum with a TIG welder. If it is going to be a rectangular style tank,
a box brake would be really handy. I have made fuel tanks with a stick welder in a pinch. A MIG machine
would have been better but I don't have one. I used 1/8 inch plate on that tank and was surprised that
there were only several pinholes to fix. Another way would be to use thin steel and braze it together using
brass rod or even brazing it using copper house wire. One could also use an acetylene torch to weld up
some thin steel to build a tank. Still another way would be to use copper or brass sheet and solder it together.
So in the end, there are a lot of ways to build a tank depending on your equipment, materials, and how
you want it to look.