I epoxy the plug in place, then drill a series of holes to accept pegs made of bamboo skewers & file the ends flush. Originally, I believe, the plug was fitted, pegged, and then sealed by rubbing beeswax into the joint.
I have generally left the plug complete, and fill from the spout end, after unscrewing the turned brass measuring spout. For the strap, I make a staple of coat hanger wire and epoxy the ends into the base, or into the horn at the base. I notice that some horns are fitted with a large threaded fill-plug, which can also be shaped to accept the end of the strap
At the neck I make a groove with that chainsaw file and fit it for another staple floating in the groove. Alternatively, a staple can be fitted into the thick part of the horn near the spout by drilling two holes longtitudinally and insert the staple with epoxy.
My main references were the GUN DIGEST series, BUCKSKINNING. I'm not sure if they are available anymore?
For a priming horn which can't be filled through the narrow end, I made the base of moose horn, and drilled and tapped it for a plug made from a moose horn tip which was drilled for a thong (to avoid loss on a trail walk or hunt). The priming horn was suspended around my neck from this plug