An older board-maker that I know showed me a system he'd been using for a long time where he'd make a timber box with a timber fin. There was a very fine tolerance so the fin only just fitted into the box. Before inserting the fin a strip of paper was folded over the base. When the board got wet, the paper would swell and the fin would be locked in
George Downing, perhaps? That is the exact description of the removable fin system he devised, in the early 1950's, in Hawaii.
SHAPER SINCE 1958
I always liked the screw from the deck idea.
So for modern fins an 8 mm rectangular slot with a mid level shelf and two screw holes.
Glass thru the board for stability, trim the deck.
Slot the fin in and screw from the deck. It could be made to fit any base shape because it’s not about the base.
Or even an 1/2 inch slot box for the fin base and two round tubes for the screws from the deck down to the box.
Downing comes to mind...
There are a few here who would remember a forum member named Mike Sabs. From Oz or New Zealand, I think.
Somewhere between 12 and 15 years ago he posted a thread about this very thing. He had a rather simple home-made system that actually allowed for adjusting the toe-in. I've tried many time to dig it up to no avail, and I just don't remember enough of it to be useful.