I have two horns, main, and priming that have " Acorn-shaped" buttons in the middle of the butt around which to tie a leather thong( latigo bootlace), or slide on a leather strap with a slit just long enough to slip over the circumference of the acorn. Use a leather punch to punch a hole at each end of the slit, and the slit will not tear, or grow. On old straps, either replace them, or run a stitch or two to tighten the slit in the strap up enough that it cannot come off the acorn.
I am not a big fan of staples, altho some are quite elegantly made. The proper way to use a staple sunk into the Neck of a horn is to run a loop Under(through) the staple, and around the horn, so that the staple is only asked to keep the loop from slipping off the Mouth of the horn, not actually carry the weight of the horn.
There are lots of ways to turn the base plug of a powder horn. Buy the Sibley Book on making powder horns from TOTW, and take a look at the pictures they have there of original horns. There are also websites showing original and reproduction horns that will inspire you.
The last thing I recommend to hold a strap is to use an eyebolt or eyescrew.
Oh, some of the acorn plugs are threaded and screwed into the base plug, so you have a much wider opening in which to pour powder when you have to refill the horn. :thumbsup: