If you install the finbox before or after laminating.
I have always installed prior to laminating and for some reason can never really get a great looking instalation. So i was thinking that routing after laminating might make ffor a cleaner final product.
We always install our fin boxes after the hotcoat. We sand smooth, then router, and install. After this cures we grind the box smooth and gloss. This makes a nice neat clean installation.
If you want to cap a box, just put the glass over the box and gently saturate the cloth with resin using a small brush. If you don’t over saturate, the resin will fill the cloth without going into the box. You can then trim the glass over the box either with a razor blade when gelled or with a trim router when finished. O’Fish’L’ does have a box that is capped which goes into the blank for a standard FU fin. I’ve had nothing but good results using that box.