I still haven't got pics of the finished forearms, but I discovered some shots of the fit piece on on my phone. As with the shins, the idea here is use a rough-cut piece of sturdy 5mm foam to fit to my arm, then decorate it with the 2mm detail pieces which are easier to fine-cut. Unlike the shin fit pieces, the forearm fit pieces are completely covered by detail work.
I arrived at this piece mostly by trial-and-error: wrap a sheet of foam around my arm, mark where it should join, then cut and glue. I was shooting for something slightly conical so I would avoid a stove-pipe look. The seams didn't have to be pretty since they'd be covered and, in fact, it was okay if I had to rip them, trim, and re-glue later. As a bonus the edges of the foam sheet made a natural "V" hollow so I oriented the rest of the design such that the hollow allowed my elbow to bend; basically the hollow is on the inside of my forearm.
Note that, before I final-glued the fit piece and all detail pieces I made sure to transfer my pattern to paper so I could construct one for the other arm.