I started making them for the same reason. Just using allen head limbolts on existing bows, I never went out and bought the metal bolts themselves. I'm sure I do it the hard way, but it works. I made several pairs using the bolts with the brass undercap (not sure of terminology). I cut a small length of allen wrench off, about 1/2 inch and epoxied it into the allen hole in the bolt. This gives the antler a bit more to grab than just the flat head. Then I use a dremmel tool to hollow out the antler butt to fit nicely over the whole bolt and halfway over the brass undercap too. Then I use an epoxy found at any hardware store that is specialized for concrete and metal and glue everthing together. Worked like a charm. Have one set on a 72# Blacktail, no problems. Like I said,. I'm sure there is a better way, but it worked well in a pinch.
I have PLENTY of antler butts here, I've been planning on posting them in the sponser classifides but have been too damn busy.
Here's a couple of pics