I did this with the intent of removing the fin for easier trasport and storage. Turns out I haven't removed the fin once, but I like the flexibility, at least in theory. A single 9" fin has made for great tracking on the 14' board. You might want something a bit different depending on your intended use but I found no cause to consider multiple fins, regardless.

A forum search should turn up some nicely illustrated threads detailing the "how to." Briefly- you'll add some type of reinforcement material where the fin box will be placed, prior to closing up the board. I used solid cedar, butt up against the final form. Route the area for the fin box slightly large and bed the box in thickened epoxy.  A block plane will bring the protruding lip of the box flush with the hull. I put masking tape just over the slot and then glassed over the whole thing. Use an Exacto knife to remove the patch of glass (and masking tape) after things have gelled but before fully hardened. Proceed with fill coats.

Good luck!

Patrick

   I searched the forum for some detailed instructions but i had no luck. If you can direct me to something that would be really helpful. 

I usually travel with two SUPs so removing fins makes it easier. I also do various conditions and have found certain fins perform better. 

   Nevermind I found it! 

Ok, great. Please share, where did you find it?

Ruud

   Found this one. http://www.clcboats.com/forum/clcforum/thread/22393.html

   I did something very similar on my board. The major difference was I started with a solid piece of balsa then cut it to size and sandwiched it between two layers of 3mm plywood to reduce the chance of tear out. A side benefit of doing the fin box was it allowed me something solid to screw into when attaching the eye strap I hook my leash onto.