Wrote out this whole thing about this… Then lost it.
Just going to post up a couple of pictures now.
This is how I salvaged an otherwise-unusable (for me) fin and a damaged futures finbox, and saved $30 by not having to buy a trailer for my twin+trailer setup from True Ames.
Well, it really just involved sanding down the sides of the futures box until it would fit into the FU box snugly. I kept the fin in the box during much of the thinning out to help it hold its shape as it got thinner. the Futures box I used was one I salvaged out of the board that I put a center box into (the one in the photo), so the top was already sanded down, but if you were to use a brand new futures box, then the little flanges on either end would be thicker and better able to support the strain.
One thing I ran into was, by the time the sides were thin enough to fit into the FU box, they were too weak to support the tension of having the futures fin tightened into the box. The first time I tightened the fin into my “adapter”, the side cracked. So, I had to roughen up the fin base with some coarse sandpaper, as well as the inside of the futures box/adapter, and epoxy it in so they could provide each other some support. So, its really not an adapter at all, but rather a modified fin/box bastardization so I could make use of otherwise (to me) useless fin and box.
If I were going to make one so I could use it as a true adapter, and be able to switch out fins at will, I would trim down the sides until there was hardly anything left, then build them back up with fiberglass, to make a stronger case. While I was at it, I would also extend the base of the box so you could set it up like a normal pin-in-back, screw/plate in front kind of longboard box mount. Actually, I’m kind of surprised that there isn’t one of these on the market already.