If mating 2 slots of equal width, why not do a flush constraint with the
edges of the slot?
If mating 2 slots of unequal width, mate the axis of the radius at the end
of one slot to the edge of the other slot and offset by half the slot width.
If mating a slot and a hole, mate the axis of the hole to the edge of the
slot and offset by half the slot width.
Does that work for you?
-Mike