: I'd recommend mixing up a portland cement-based mixture, rather than using
: your expensive epoxy....but if you really want to get rid of epoxy, then
: just try straight silica sand.

I agree with Shawn about concrete being a good and low-cost solution. If you really want to use epoxy though, you should think about epoxy mortar. To get the biggest bang for the buck, you need to minimize the volume of epoxy and maximize both the volume and strength of the filler. Think about how you would fill a large volume with balls. You might start with beach balls, but the spaces between them would be about soccer ball size. Add them, and now the spaces are maybe softball sized. Then ping pong ball size. Then marble size. Then BB's, ad infinitum. A graded mix of particle sizes most efficiently fills any given volume. Additional benefits are lower epoxy usage, and the aggregate tends to moderate the exotherm generated by the epoxy (preventing overheating, cracking and degrading ultimate strength.)

This is the same problem as a trowelled epoxy floor, and here is a good recipe:

1 part 16 mesh silica sand (angular particles, clean, uniform gradation)
1 part 20/30 mesh silica sand
1 part 70 mesh silica sand

Don't put in any soil, clay, wood flour, or anything of that nature, they will will develop shear planes that will degrade the ultimate strength of the material. Fiberglass shorts would be OK, but they won't have a measurable effect on compressive strength. Premix the sand in a volume slightly exceeding what you need, and add just enough epoxy to wet out all the sand. Pack it into your molds (did I mention that you'll probably have to cast these buggers?) around the mounting bolts, tamp and trowel it, and there you have it. With any good construction epoxy you will have a material with 12,000 lb. compressive strength, easily three times that of concrete. Odds are they'll look like rocks, too!