It looks like you've shimmed it. Tapered shims might make that easier and it seems to be a reasonably common solution.

I'm taking a different route. Instead of locking casters, I'm going to put lifting casters. That way I can deploy the casters when I want to move something and drop to standard levelling feet when I'm in position.

Lee Valley and others sell them, but there are a variety of ways to do it yourself by mounting the casters to something with a hinge, then lever them up into position.

I've also seen methods for mounting a couple of narrow, wide boards on a dolly, sliding it under, then twisting those boards upright to lift your equipment. One dolly (or two for long things like a workbench) will do for everything and it (they) don't even take up space because you just store them under your cabinets.

Nice cabinet. That's the direction I'm taking with my drill press, bandsaw, and press. By my calculations, that plus storage in my workbench should free up enough space on my walls for some basic lumber storage.