If the fin fits too loose, the fix is to use a shim. This is an easy fix. At our shop we use some packing tape and fold it on itself so no sticky tape is exposed. You can adjust the thickness of the shim by using more folds if the fit is very loose or less if you just need a little shimming.
Put the shim under the base of the fin and push it into the finbox as you push down the fin, like in the pictures below, you should have a tight fit with the shim, no wiggling. If you still have some wiggle with the shim, pull it back out and add a couple more folds of tape to the shim, if it's too tight, make a thinner shim with less tape.
If the fin is too tight and won't go into the box, you can carefully sand the sides of the fin base to make it fit, this takes a little bit more time and patience. Carefully check where the fit is tight and slowly sand the area down until it fits tightly. Often it is the bottom of the fin that is hitting the bottom of the box. In that case, you have to sand the bottom of the base to make it fit, not the sides. The other option is to find a fin that fits the box without sanding.
Another tip: You can use the fin key used for the side fins to put the fin plate into the box and to line it up with the screw hole so the screw goes into the plate easily. That's it, enjoy!