Having built a base, or two, for racing seats, I'd say that 1/4
material is overkill. 1/8 material is sufficient and is what all of
the commercial set bases that I've seen are made from. Pretty easy to
bent 1/8 material by clamping it in a vise and whacking it with a hand
sledge. No heat is needed, nor is a lot of precision required.

1/8 should be good enough and it should be much easier to work with.

Thanks,
Freddie