Al, you beat me to it. I'm lucky - I have a reamer that was my dad's. It is about 4 1/2" long x 1/8" at tip and 5/16" at the handle. The handle is a small Jacob's chuck. Cuts like a dream and makes a nice long taper.
I've used fiddle pegs for stoppers lots of times, and currently have a favourite horn that uses one. But the best horn plug is made from pine or spruce, or maybe basswood, poplar, or cottonwood. It should be of a soft wood that will compress and expand. They stay put in the horn much better than a hardwood peg, especially a peg of exotic wood like ebony or rosewood with their natural oils that work against you. A soft peg will have less of a tendency to split the horn too.