RE: Tuttle fin in Deep Tuttle box
Perhaps your std. Tuttle fin root didn't fit the fin box really well. If the fin root did not make full contact on the front and rear tapers and fit very snug on the sides (and it sounds like this was your situation as the fin was going too deeply into the fin box so you could never really tighten the screws up correctly to the point that the fin was "wedged tightly" between the front and rear tapers at the correct height).
Anyway, not to worry......!
A good repair shop can fix your existing fin box fairly easily.
A demo customer ran the 75 cm fin on the Apollo I had into a tree stump in a lake at full speed.
It moved the fin back in the box almost 1" and bent the fin screws beyond my ability to straighten them.
Donnie Bowers at Fox Watersports was able to fill the back of the fin box with a thickend epoxy slurry with some glass cloth in it and then he put the fin back in with a mold release compound on the fin root.
Fin now fits better than when the board was new.
So, get some thickened Epoxy (JB weld works pretty good here) and some wax paper or mold release compound (even a thin plastic bag would work) and fill the damage in your fin box with the epoxy mixture.
The put the fin root in the box (leave it about 1/32" proud of the bottom of the board) and when the epoxy sets up your fin box will be as good as new with the front and rear tapers restored.
Don't get the epoxy all the way down where the top of the fin root engages the tapers, but try to get it so that perhaps the last 1/4" is free of the epoxy mixture.
After the epoxy has fully cured, take a round file and "fit your fin" as I suggested earlier, except now you will be working inside the fin box more than on the root of the fin.
Hope this helps,