Incidentally, I made a frame for the seat out of old bed frame rails to hold everything in alignment and keep the boards from warping. I don't know what kind of steel they use for those things, but it is HARD and BRITTLE.
I had to drill four 1/4 inch holes in the middle piece, and I ruined two drill bits. Neither one could do more than just scratch the paint (and I'd been using them for drilling all the other steel in the project with no problem). Wound up using a die grinder to bore the holes and that took a long time.
Then, after it was welded together and installed, I sat down on it before installing the wood and the rear angle cracked right in the middle (and I'm not that heavy). Anyway, had to weld it back together where it cracked. Weird stuff...