I have a black horn waiting to do what you have. I'm not sure what the next step is. Do you fill the inside?
Assuming your horn is dried, cured, and no longer has the core the first thing I do is sand it smooth starting with a dremel for the roughest parts then sandpaper, 80 grit going up to 220 grit or so. Then I cut it to length and with the angles on the ends I want. I use a table disc sander to get the end of the shank and horn flat and able to sit flush with each other. You could use sandpaper and a sanding block, it just takes longer and you need to be careful to keep the sanding block flat.
I use a 7/16 threaded rod to attach the horn and shank. That maybe a little overkill on the size of the rod but it is what I have on hand. Depending on the length of horn I use a 4 to 5 inch piece of rod. Drill a hole half the length of the rod into the shank. I use a little 2 part epoxy in the hole to make sure the rod is in as solid as possible. When that is dry I place the small end of the horn over the rod making sure the horn and shank are flush. I'll pour in fiberglass resin filling the horn to the top of the rod. After that sets up you could fill the horn the rest of the way with bondo, fiberglass resin, or other products. I use wood dust and shavings (we all have plenty of that don't we) and pour in fiberglass resin to soak in. I let that set overnight.
I'll trace the shape of the top of the horn onto a small piece of pretty wood. Cut it a little oversize and glue the cap to the top of the horn after sanding flat so it sits flush. Then using a rasp and files take the wood to the same size of the horn. Before shaping the top of the wood I drill a hole in the middle through the wood into the horn filling. Keep in mind the threaded rod to avoid drilling that far. Shape the cap with a dremel, rasp, files, ect... I use a plug cutter and cut a plug out of the scrap horn (or a piece of wood). Then I drive a short dowl rod into the hole and place the plug on top of the dowl rod and drive that in.
With the horn handle roughed in I sand everything smooth to 600 grit. After that I use a polishing wheel to shine it up but you could use rubbing compound and a rag.
This is just the way I do it. I have seen a few other variations such as having a plug on the underside of the cap that is glued into the horn. The advantage of that would be less wieght but I have a hard time having it fit snug and flush. I started using the dowl rod and plug in the cap after my mom knocked the cap off the stick I gave her. Not sure how many times she dropped it on a concrete floor but I know it was more than once.
If others have better ideas I'd like to hear them. I have some ideas on how to leave a small storage space and a cap that slides open in the middle but I'll keep them to myself till try it and work out the bugs.
