Infected by a random appendage:
As I mentioned earlier, the forearms are made from a fit piece covered by details (mostly) made of 2mm craft foam. I kind of made the right arm up as I went, cutting and test-fitting pieces and pre-assembling them using masking tape. I made alignment marks (you can see them all over the pics) so I would remember how to re-assemble. Then I disassembled, copied the pieces to a pattern, used the pattern to cut the left-hand side pieces, then assembled both forearms with hot glue (the high-temperature stuff -- holds way better than the low-temp variety).
The white chalky stuff is regular construction caulk, used to fill the seams.
The multi-colored assembly looks tacky, but it was a huge help visualizing and assembling the bits. It'll all go away when painted, of course.
These were actually my fourth (!) attempt at these. The first three (constructed in 10mm, 2mm, and 5mm thick foam, respectively) were based on a Pepakura file I found on the Internet. No matter how I tried, though, I just couldn't get the scale right for my arm. Hopefully lessons learned here will keep me from that many tries on other pieces.