Yep, I concur with what's been said. Sometimes, with antler and hoof overlays, I'll intentionally leave the piece bigger than the finish piece will be, just so I have something to hold on to. Then I trim off the extra once it's level.
With smaller pieces it can work well to clamp my farrier's rasp or file into a vice and rub over it, instead of the other way around. Just be careful or you'll have fingernail on the rasp along with the antler residue. (I know. I've done the research)
Has anybody tried boiling the antler piece, then smearing it with thick hide glue and squishing it into place with a bar clamp? I've gotten good results doing that with hoof overlays. I wonder if it would work with antler/bone.