Regarding Tectonics fins, I've got six of them with Tuttle bases and I've never had to sand a single one. They have always worked perfectly with no modifications needed. I also find True Ames fins to be right on with the tolerances of their fins. In addition, I have a number of Lessacher fins too, and they all work well, except one is a bit on the tight side, but it still fits.
So all and all, my experiences support a position that fin manufacturers are doing a great job with their tolerancing. So, I must consider your findings to be very unusual, at least with Tectonics fins. I could be mistaken, but isn't the F2 brand using Powerbox in their small and medium boards with cavity type boxes?