I riped the fin box out of my long board. To put it back in I epoxyed the box into its hole. I filled the box with popsicle sticks and put a ton of mold release wax onto of the sticks. When the area was all fair (I had to fill the area around the box with micro balloons and epoxy to get it even with the height of the fin box). I epoxyed two layers of fiberglass over the entire area. and extended the patch bigger then the fin box itself, this makes the repair stronger. I then vacuume bagged the repair, but you don't need to if you can get the patch smooth.
after the repair had completly dried, I cut out the repair job, just in the area of the fin box where the fin goes into the box. The point of mold release wax is that the epoxy or resin of any kind will not stick to it, so you can cut it out later on. If you have any questions you can email me at firstname.lastname@example.org