These are side fins for a longboard in thruster configuration (3 fins). These fins are made to be compatible with fin boxes made by Future Fins. But they can easily be made compatible with FCS fin boxes by cutting a notch in the tab at the base, or glassed on if the tab is fully removed.
These fins are specifically designed for a longboard, but could also work as side fins on a quad or to make a shortboard trailer fin for small days.
Also check out my shortboard fins at thingiverse.com/thing:12495
and my londboard trailer fin at http://www.thingiverse.com/thing:12496
If you like these checkout mesheldrake's Cardboard Surfboard thingiverse.com/thing:4764 or his other boards at sheldrake.net/cardboards/
His most excellent work on these cardboard surfboards inspired this creation.
For side fins use 1/4" or 6mm stock and cut one for each side, flat side faces in toward the center of the board. For the center trailing fin use 1/8" or 3mm stock and cut one of each then glue them together. Set red to vector cut and black to raster. The foil will be made by the black raster, so run it at full power and very slow. I used a 60 Watt Universal laser cutter with power at 100%, speed at 1.5%, and 1000 ppi for 1/4" birch plywood. With a little sanding and maybe some epoxy or polyester resin you can make and surf your own fins.