I've heard of people modifying fins to make them smaller but recently I wrote off two fins but I have a third that I can use but it is too small for the sails I want to use it with. Has anyone tried to increase the length of a fin and had any success? Any ideas how it could be done?

I've seen a fin with small carbon splints in the tip, holding on to a small fiberglass extension.

I've had success myself by grinding a taper on the tip, that extends back 30mm-40mm, then building up with cloth to the required size.
Don't think I'd want to go more than 5cm or so though

Surely you could just get a 2nd hand fin for like $50-$70?

Wouldn't that be a lot easier and stronger.