Did you ever get the 52 cm st. Tuttle base fin to fit correctly in your iSonic 145?
I'm wondering if your finbox had a "lump" of epoxy on the back taper to "set the
Unfortunately, the folks at Cobra sometimes do the "glob of black epoxy" on the back taper of the fin box to make a fin box that's out of spec. (the fin goes in too far) work better with the fin.
This results in a really small area of contact on the back of the fin and yes, the fin can be "stuffed" up into the fin box by pretty minor impacts.
Only way to fix this is to grind or file out the "lump/glob" and carefully use either mold release compound or wax paper to fill in the entire back edge of the fin box so the tapers engage correctly.
Hope this helps,