lezo thanks for the explanation and perspective. The fuse and leash seems a smart idea, especially since it works for you in practice!
I still think a lever as long as the hydrofoil could put a lot of torque and stress on a part the size of a tuttle box. I would think this stress could be reduced by using a tapered composite plate on the foil, mounting to a flat plate in the board, with the bolts set in from the edges and corners a bit.
The maximum moment would be at the bolt position, instead of the edge of the plate. The taper of the plate could allow some flex to avoid putting so much stress at the edge on impact.
This way perhaps excess stress on the materials could be avoided.
Such a system could still be fused, by washers as you did, or some other way.
Anyway it sounds like your solution is smart.