The trouble with homemade steel gas tanks is that they rust up on the inside. I made one once. Worked great for about 18 months. Then the fuel pump wouldn't pull gas out of it any more, so I dropped it and looked inside. The whole thing had rusted up inside, and the in-tank filter was clogged solid. You have to use "aluminized steel," whatever that is, or stainless steel or aluminum. I wouldn't know where to get that, or how to weld it. Maybe you could just paint the inside with some kind of gas tank liner, also. They make special stuff to fix antique cars with rusted tanks. But who knows?
Aero can custom-make anything you want, and theirs won't rust, but you're talking fairly heavy bucks.
Chas Langelan, Mt. Airy, Maryland.