I just had it upholstered there, I built the pan and glued/shaped the foam myself. I used an off-the-shelf piece of 16g steel from Lowes. I believe it was 8x24. The rear section of the frame is 8 1/2 center to center, so the large rear hoop from DCC works pefectly. The 8" wide sheet gives a nice reveal of the frame/hoop, and you only have to cut and shape the rear half circle and the front section where it narrows down to the tank.
Oh, and it's mounted to the frame via two holes I drilled in the crossbrace. I used threadserts (threaded inserts installed with a riveter) in the seatpan, then ran two bolts through the holes in the crtossbrace and into the seat pan. When I bent the pan, I left the bend s a little shallower than they needed to be, which means the screws force it down flush with the frame and rear hoop so no other mounting points are needed.
Hope that helps.