The local rock gym has an employee who makes holds. He's experimented with several materials. Wood does not make good holds, due to cracking and splintering and relatively low friction and wear resistance. The holds he makes now are a combination of resin (polyester, I believe), chopped glass fibers (for strength) and coarse sand (for texture). He mixes it to a dough-like consistency, shapes it to his liking, impresses a steel washer into the surface deeply enough to recess the head of a 3/8" allen head bolt, then pokes a hole through it. Once fully cured, they work great, provide excellent friction and wear like iron.