have you tried the shoulder filing jig on the guard? Once you drill the holes for the slot and drill them out and get rid of the webbing in between, Line it up where you want to file to and file down to the guard, since the jig is hardened you cant go any further, good and flat. I just got my shoulder filing jig from Koval Knife Supplies and did this on the one i am working on, worked good, very square slot. I think Bruse Evans mentioned doing it this way.