Hey Nessmuck
Didn't know if you found what you were looking for in a horn. Not trying to toot my own horn here,
, and I'm certainly not one of the "go to guys" listed, but I do make and sell powder horns, and have a large right side carry plain horn that would be good for a smoothbore - that uses more powder than the typical rifle, And it is generic enough to fit in several timeframes. Its 17" around the curve, has an octagonal tip, poplar base with staple, is lightweight for its size, and has been lightly aged. In case your still searching, I thought I'd share this one for consideration. I'd take $130 for it plus $10 shipping. Bigger horns like this are getting harder to come by. You can PM me if interested. If not, happy hunting!
Brian Huffman