I have used epoxy to fill voids in white ram's horn, and it worked out ok-ish!
The OP's piece of horn is black. Not sure if you can get black resin, but you can use powdered horn as a thickener, so that might work.
Sometimes, though, the greasiness of the horn prevents the resin from sticking and curing properly. Particularly in the centre part of the horn where the core was removed.