After using and wearing out a few file guides I have two suggestions.
Buy one of Bruce Bumps file guides. The have two points of contact and will tighten down evenly.
Two,
Just take two 2 1/2" x 1" x 1/8" or what ever thickness stock you have and clamp them down evenly with one of those 1" C-clamps. After each knife, true the edges. Or just buy one of Bruce's Carbide bad babies and be done with it!