Here's what I did (I am no great horn maker mind you).
I made a wooden cone as a mold to make the horn a concentric circle heated the horn with oil and shoved the cone in. Then I made the plug. I have a crappy lathe but it's a lot of work to setup, so I did it by hand. I started with a 3/8" step in the plug to go inside the horn. I cut that with a Japanese pull saw and chisel. I sanded the horn rim flat on a piece of sandpaper on a table and very carefully tapered and reduced the insert part of the plug until it fit.
I had to trim the edges of the step with the chisel until it fit perfectly. ( I made a pencil mark on both to keep them lined up because the horn still wasn't a perfect circle). Once I had that, I traced around the horn into the plug. Removed the plug, trimmed down to the line, and shaped it. When. I was happy I glued and tacked it in place with brass brads.
After all that the wood was still just slightly larger than the horn. I used a steel scraper to make it even with the horn, but probably would have been fine like it was. Not a 5 min job, it took a full evening. The lathe would have definitely been faster.
