I have one board with a power box and 5 fins. 2fit great, one disappears down into the box and the other 2 sit proud.
i have 2 boards with deep Tuttle boxes and 7fins. 2 fins fit one board perfectly, 3 fin fit the other board perfectly and 2fins don't fit either very well. One board the box is slightly narrower and longer than the other board.

so my winter project is to get all the power box fins fitting the one board and all the Tuttle box fins fitting both the other boards. The power box one is easy a bit of sanding on one fin and build up the sides with some epoxy on the other. the Tuttle boards are the tricky ones, for the fins to fit both boards I'm going to have to make one box narrower and one box shorter, not sure how I'm going to do that yet.

For building up, just resin flakes off

glass tissue or a 30-50gsm ish plain weave is your friend.

Yep I've got some glass to use as well.

Super rough sand your glassing area. 60 grit. This will only work on resin heads , ( which most are ). I don't know what to do with plastic heads.

Any ideas how I make one Tuttle box shorter by about a mm and the other one narrower by a tiny bit