Surfing School - Glassing - Fin Box Hole
Place the fin box on the edge of the template cut and with the 1/2 inch diameter bit in the router (the router will be upside down with flange directly against the plywood), and adjust the bit to a distance about 1/8 inch beyond the finbox. Measure the board's tail section to ensure the blank is at least 1/2 inch thicker than the installed box will extend. I sometimes sand a finbox down to fit if a tail section is too thin, but don't advise this for non- professionals.
The distance for the bit extension is critical. Once adjusted, make sure the router flange is tightly secured to the router or you could have the bit go too deep and cut through your board.
With the board on its top on your sanding rack, lay the finbox on the tail section and make two small lines (top and bottom of the finbox) with a pencil. Make one last eyeball estimate back from the tail section to ensure the box cut will not go through. Next, lay the template over the tail section, lining up the middle marks with the stringer, and over the marks you made on the board. As a final precaution, tape the top and bottom of the template to the board to ensure it doesn't move. You're now ready to cut.
Place the router bit in the center of the template over the stringer, and cut straight down until the flange meets the template. From that point, move around the template until the hole is cleanly and evenly cut out. Remove the template and put one end of the finbox into the hole to make sure it will fit and not be too snug.
If the bottom of the board had a vee shape or was not flat, you must be careful to ensure that the template was exactly level so the bottom of the cut was horizontal. Taping the template securely and level first will prevent an uneven cut.