I made my own fins doing the following:
1. Pulled a fin off my current board (1.5" deep).
2. Bought some modelling clay from Michaels.
3. Put modelling clay in a container and pushed it down. Made the surface flat as possible.
4. Put cooking oil on the fin so clay wouldn't stick to it, and pushed the fin down into the clay. This makes a crude mold.
5. Poored epoxy resin into the mold and let it harden.
6. Once hardended, I pulled the new fin out of the clay, cleaned it up, and sanded. Pulling the fin out distorts the mold, so I made a new mold for each fin.
7. Then, drilled and tapped the holes for screws.
I spent $12 for the epoxy resin at Lowe's and got 6 fins with some resin left over. The fins came out slightly thinner than the original. I'll tweak this a little next time.