Just another fin jig. Actually two jigs - one small, one large, plus an adapter sleeve.
It has slots for both 3 and 4 fins, with a cutout that allows a motor clip to poke through.
Small jig fits into 18mm (BT-20) motor mount, and is spaced for fins up to 3/32" thick, the most common.
For thinner balsa, just add some card stock to hold.
The sleeve fits over the 18mm riser and lets you use the small jig on a 24mm motor tube.
Larger version is 4.4" diameter, and designed for 1/8" fin stock. It's built to fit into 24mm motor mount (D/E/F), and can be used on larger body tubes with a greater fin span.
These jigs work best with fins that extend below the bottom of the body tube.
I have used the small one, but not the large one.