Not exactly sure I follow your logic about creating an index surface.
I only touched the slot corners with the file lightly. More just to
make sure there were not any dings or blemishes to effect the caster
movement than to create a bevel. I can't imagine a moderately used
sled ever causing any noticable wear particularily since there is no
friction. The caster rolls over the top of the slots, it doesn't
slide. Seems counter-intuative to me they would cause more wear than a
tightly fitting metal bar. Perhaps I just dont understand what your
referring to. Mike