Any good carnuba wax will work as a mold release...although your comfort level will certainly go up if you get a wax specifically made to be used as a release.

I used milled glass fibers for the filler because it really sets rock hard (chip it with a hammer like). Coloidal silica will work fine too - micro balloons might be a bit soft and not resistant to abrasion (if there is a little wiggle).

I kept the resin out of the bolt holes by putting a clay ring around the holes and a clay dam at the ends. That first-grade silly putty snake making method has come in handy once again.