Most of the Commercially made Tail Mounts are basically a 'Badge' shape just about 6 inches wide and about 6 inches high, with a hole drilled in the center (bottom) for the Beard.
If you want to make a full sized backing for your tail, it might be better to dry the tail first in full display and take measurements from that, leaving 2 or 3 inches extra wood all the way around. Again with a hole drilled in the bottom center for the Beard mounting.
When I had it done, I just made the wood 'custom fit' fan shape out of White Cedar, with a little wooden protrusion at the bottom center for the beard. A cheap Tin retainer was used to hold the tail to the wood. The beard was stuck in a 3/8" hole at a 45 º angle and the spent shotgun shell to the side. In other words, it might be better to customize it than to look for a 'one size fits all'.
Stained and using Poly Urethane, they come out nice.