One for the brains trust...

Got a tuttle box fin that currently sits too deeply in a deep-tuttle box. I've no idea if the fin head us undersize or the box is oversize, but it's sitting about 1mm beneath the board base.

Any tips or tricks for bulking out the fore and aft end of the fin head to bring it up flush with the bottom of the board? (The bulk required is too much to be accommodated by just some layers of tape).

The bulking out with fibreglass and resin (potentially in several steps) is not complicated, but I'd love some wisdom on how to maintain the right radius and straightness of the bulked out areas to ensure nice, even pressure distribution and transfer from the fin to the box. Are there any moulds or jigs that are useful or does one just bulk it out roughly and sand back carefully by eye to match the box shape as best as possible?

Thanks...