To bevel a fin free-hand, assuming the bevel isn't too acute, this is the best method that I know of.
- Lightly spray the back of a sheet of suitable sandpaper with 3M spray adhesive, and press it down on a flat hard surface, like a piece of particle board or finish plywood.
- Hold the fins you want to bevel in both hands, raising the back edge (the edge opposite where you want to bevel) to the proper angle with your thumbs, and spreading your fingers across the edge you want beveled to provide even pressure along that edge, push the fin back and forth to sand the bevel. Maintain a constant angle while sanding, for a flat bevel.
I've done this with pretty decent results on balsa, if should work on thin, lite ply, as well, with the proper grade of sandpaper. Leave a few inches of bare surface in front of the sandpapered area, so you can rest your thumbs there and not sand them raw.
If you're working with thicker ply, or tough ply like birch, this method probably won't be very practical.
A spacer block, underneath the lifted fin edge, may help to ensure consistency, as well (instead of just eyeballing it).