Finally found some time to (re-)build homemade FCS weed fins using Clay's technique:
1. Cut out a mould using slippery plastic (milk jug) with 40° slope and join sides with duck tape
2. Roughen fin surface (leading edge) with sand paper and clean with isopropyl alcohol
3. Fill mould with 5-minute epoxy ($1.50 from Dollarama)
4. Wrap leading edge of fin with mould and epoxy to extend it
5. Allow epoxy 5 minutes to harden before removing mould
6. Allow epoxy 24 hours for curing before use
7. Remount extended fins on surfboard
8. Fill air gaps between weed fins and surfboard with Marine Goop
9. Smooth down leading edge with sand paper