There are some things that don't change much from one era to the next, but youj gotta realize that these people were just as much into style then as we are now! There are a number of details you can glean from websites like Contemporary Longrifle Association (serious skills in this crowd). They have actual antiques as well as contemporary work.
For horns, search out Roland Cadle, Art deCamp, and Joe Becker. These three guys work elevates powderhorns to serious collectable art. Look for the Honourable Company of Horners, an organization dedicated to the Guild of Hornworks. It's much like the old guildcrafts, you must apprentice under a Master, pass his testing to make Journeyman, then continue your own craft research and training until such time as you feel ready to submit a series of "Masterworks" for inspection by a cadre of Masters who will judge your work. These guys are pretty serious about their research on historical pieces.
Then you gotta consider what socio-economic level the person would inhabit. Too many "commoners" carry super-high grade curly maple custom engraved flintlocks when they should be carrying a used up, passed on musket of questionable safety. Then consider their location. Many horn styles are unique to regions. Also remember, it was more likely for a rich guy to carry a plain horn than a poor bloke toting an engrailed, banded, silver mounted, three color scrimshawn powderhorn with handwoven 4 color silk shoulderstrap!
And whatever you do, if he is going to wear it at Rendezvous or re-enacting DON'T ANTIQUE IT! Nobody walked around in the 1840's carrying a powderhorn that was 170 years old.