Set up your easel at the largest print size you make, square the blades and then make or affix reference marks along the length of the blades, e.g., paste a mm scale on the blades, or whatever. Then check the blades at several different sizes with and without squaring them. It will become obvious which reference marks (and how many apart) you need to move the blades to for square. Then you can use magnets or tape or whatever to keep them in place.
Or, if they are just a bit off, and you use the classic dry-mount presentation, you can just square up the prints when you trim the borders off.
FWIW, all the two-blade easels I've used (Bogen, Saunders and a couple others), had a way to square the blades. You might double-check that. Or maybe you can improvise some shims, etc.
Best,
Doremus