Adjust fin to fit box?

should be "snug", neither tight nor loose. So the problem is the variability between boxes and fins. You make it right for one board, it'll be wrong for another. It's better for the fin to sit a little low than a little high though, so adjusting for the tight boxes is the way to go.

The front and rear faces of the fin head should be dead straight or slightly concave, if they are convex the fin can pivot more easily, same really for the side faces.
I use a small sanding pad, so I can work on the middle without taking too much off the edges.
You can get an idea where it's tight by rocking the fin, it pivots at the tight spot. Use a straight edge to check for convex faces, sand the high points down.
If I have a sloppy fin I build it up with bog and sand it back to size. The other way is to liberally apply release agent to fin box, coat fin with bog and fit in flush.
And just hope you got the release agent right!!!!!

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Pipington said..
Thank you. What is "bog"?

It's a panelbeater's, board-fixers term used to describe thick sticky stuff you can bung in dents. the sand down smooth when it sets.

Commercial panel beaters "filler" is usually based on polyester resin, which will melt the foam in your board, so it's a good idea not to have this in your shed in case at some stage you're tempted to use it.
I make my own with epoxy resin and qcell, but you can mix flour with arealdite, or use that stick of epoxy that you break the end off and roll around to mix, sorry this old brain is having trouble remembering stuff.

Thank you both!