If anybody here is into carving and scrimshawing their own powder horns, this is a very nice book.  It has drawings of historic horns from the F&I era and American Revolution. The horns are depicted as if it was rolled out flat.  You can xerox the image and adjust it to fit your horn, glue it and then scrim shaw it.

Don't want to leave you hanging here so I will post that I also have the book. I am acquainted with some of the minds behind its publication as well.  I've referred to it when looking for ideas.
The only thing I caution is that the illustrations are renderings, not exact copies, of the designs on the horns, and there might be a little variation between the drawings and the actual scrimshaw in some cases.  You can see it in the book when photos are included next to the renderings. That doesn't really bother me however and I'm thankful that Hartley took the time to do the study and that the HCH decided to publish this great book.