I had some spare model aircraft plywood laying around when I considered mounting my foot rests (they're great, thanks Jeff!). I just cut the plywood up into two 1" wide by 4" long sections, sanded the edges down, and drilled a hole for the mounting bolt.

Lashed the plywood to the frame, and it's held so far. I figure if it'll hold the frame of the kayak together, it ought to last the foot rest at least until I reskin.