I have a friend who's into rock climbing and is making himself a climbing wall in his basement. He has only one 4x8 sheet of plywood on the wall with a very few wood blocks for hand-holds, because the purchased holds are expensive (~$7 each, and you need quite a few...)

I have a bit of extra epoxy and was thinking about attempting some holds. Any opinions on what would be the best for cheap, bulky filler? The holds would be in irregular shapes cast around something like a big hex bolt for attaching to the plywood climbing wall. The holds have to be strong enough to support a man's weight.

So should I keep with the standard sawdust? Sand? (it would mimic rock nicely)