Bought a Bunger board at a garage sale locally. Think it’s from the late 60s, single fin. Had some major repair work that was needed and had done professionally. Long story short, tried to replace the fin with something more modern (Greenough 4a, and found the base of the fin is too thin for the box. Looks like a standard longboard box to me but who knows. The end result is the fin slides around in the box and won’t stay in place even if screwed tight. Any suggestions on how to fix this (outside box replacement). See pix below of the fin it came with and the box. .