The Book I meant to refer you to is one on "Recreating the 18th Century Powder Horn", By Scott and Kathy Sibley. My apology for the mis-spelling. It is still available from Track of The Wolf, and other suppliers that deal in books.
What you do with that piece of horn is entirely up to you. YOU CAN try to copy some existing style of horn, using these references, or you can do something of your own.
I rejected a Long, Big Horn early in my involvement with Black Powder after examining some that were owned by other club members, who were carrying a couple of pounds of powder in the horns strung over their necks at the club shoots! Our matches only involved 20 shots, so I thought what they were lugging around was a bit much.
The more I hunt, the more I prefer pre-measured powder charges carried in individual containers. This due to weather, and negotiating heavy brush to get into and out of the woods to a stand. I do own and use a medium sized horn that carries a little more than 1/2 pound of powder. Frankly, other than when shooting my shotgun at the range, its still more powder than I need in the woods.
So, I have some small horns I bought at Friendship, that I hope to flatten, and make into small horns, that can be carried in a pocket. If they carry 300 grains of powder, That will probably be too much. But in a shirt pocket, I know the powder will stay dry, be readily available for reloading, even when I am squirrel hunting with my .36 rifle, and well carry enough powder for a fun day within the daily bag limits on Squirrels( or rabbits). I bought more than one horn, so that if I screw up- a normal expectation- I can just begin with another horn. If I am successful, and make more than one of these horns, I can make a gift on one of them, or sell the others.