If the other board is really snug in the casting then try exchanging the boards. My experience is that both casting and boards were build to different sizes over the time the Prindles were build. The newer boards were just slightly thicker. This trick may get both your casting and boards to sit relatively tight. On prindles how ever you never get a perfect fit. I always solved the remaining slop by using washers.

(owned a 1975 Prindle 16 and a 1986 Prindle 18)

Wouter