A while back, I was doing some research on how to make a powder horn for some blackpowder shenanigans, and came across this~!
http://www.armourarchive.org/essays/ess ... horn.shtmlI found it moderately helpful, as it can easily function as a powder horn as well, with a stop- cog and all. I used a buffalo horn I bought at my local leather store (way back in Santa Cruz *sniff*). I didn't do any etching, since the horn was jet black anyways, and it had a fault line down one side. If I do another one, I'll make it from bull horn, so that I can be all artsy with colored inks and what not.
I've heard the best horns for trumpeting come from Aurochs. Good luck finding one of those~!