I use EMT conduit and weld it with a wire-feed welder using flux core wire. I use 3/4" conduit for the roll cage and the main tube of the front axle. The rest is 1/2" conduit. The main side rails are triangulated for strength and rigidity. The body sides are .025" aluminum "kick panel" normally used in screened porch installations and the tops of the nose and tail are made from .060" polycarbonate plastic (Lexan). I cut them to fit and rivet in place, so they become structural members and add to the overall strength of the "tub".

EMT conduit is, of course, just mild steel thin wall tubing that has been zinc coated. It welds best with flux core wire. I have used regular MIG on some of it in the past and the zinc splatters and interrupts the arc. The flux core wire welds more consistently because the spattering zinc doesn't affect the arc.

I have built 18 cars since 2004. A couple of them have taken violent flips and the roll cages have held up fine. Both of those cars, after some repairs to other components, are still competing.