I bought some owl pellets for the mice bones inside them from a local aviary (you can get them online) to mod the bases for my nexus and I'm pleased with the results.
All I had to do was get the pellets wet to loosen them up, pull out the mice bones, and let the bones soak in hydrogen peroxide for a couple days to sterilize them. This step also bleaches the bones, so when I fixed them on the bases I used a thin layer of agrax earthshade to grunge them up a bit more. I harvested roughly 10 bones or so per pellet and now have lots leftover for future beasts. I thought about putting some blood effects on the bones but figured that it might be too gruesome for play with the family.