Making a tapered hole, or even a rectangular hole, is pretty simple, really, especially in antler.

You go to the hardware store, find the sawzall blades, and pick out the one with the nastiest sharpest teeth you can find. Then you go home and fasten it into a handle. If you have a grinder of any kind, grind the non tooth side off so the blade is only about 1/4 inch wide. You start with a round hole in the center, then use the saw blade with a pull motion to enlarge it in any direction you want to. Mine has a stag handle, like my files, and many of the knives I make.

If you don't have a grinder, just buy a sawzall blade that is already narrow. And if you can't/don't want to fool with making a handle, get a long one, and wrap duct tape around it.