All jokes aside, regarding sloppy boxes!!

The el-cheapo solution is just layers of Electrical Tape on the sides of the base of the fin... But you only get 1 shot at it. For a tight fit, the tape gets a little squished, so if you pull it back out for a second go, you need to re-do some tape...
So you need to know what position you want the fin, and have the plate carefully positioned... or use a "fabricated paper clip" to help position the plate... or similar...
I have a board with the SLOPPIEST BOX EVER!!! 3-4 layers of tape on each side.
PS - it's not a McTavish... All my McTavish boards have top quality fin boxes which are absolute perfect fits for the fins.