Haven't done a "strong box" yet, but I have heard that tail rocker and V can be an issue. Looks like you compensated for it nicely. Lowel
It took me and my surfboard building buddy Rex about 4 hours to dial in the router and future center fin jig.. This one came out the best. We drop the fin box just a hair below the foam. we then fill in the flange with resin and millled fibers.
I saw a trick on Josh Martins IG where he used a heat gun to bend to the rocker of the tail. It works. I always have trouble with boxes and tail rocker. The heat gum is a good solution.
all the best
Personally I'm always ready to learn, although I do not always like being taught. - Winston Churchill