Lots of photos of them over at the muzzleloading forum, including ours.
Ours are more for shooting than display. The bases are made from 1" (3/4") pine, 3-1/2" - 4" wide and 10-11" long. About 2" from one end a vertical piece of pine the same width and about 4" high is glued and screwed. Before installing however, it is cut out in the center with hole saw and a little further down for the hammers. On the base, about 3" or so from the vertical piece an indentation is made into the wood for the stock (grips) to dig into. We also add another piece of wood with cut outs for lead and wads.
There are plenty of plans on the web, lots of variations. My girls like making them, they're simple and quick to make.