Was reading Moorehead's posts about box joints but the
Google got confused and wouldn't let me post reply.
I had a similar problem with my home-made jig. My cast iron Delta
TS has very tight miter slots but I found that if I applied side
pressure to the jig (i.e. press jig in the slot, applying
pressure toward the blade in a consistant manner.) it took out
the slop and made the joints perfectly. Perhaps this is your
solution, dunno.
Roger