A simple and cheap alternative to rubber-feet, and which you might already have around somewhere, is just a couple of strips (or squares, if only small areas are used as supports under the easel) of flat self-adhesive foam draft-excluder. That would be fairly grippy on the enlarger baseboard. [EDIT: even cheaper and more easy to find. get some masking tape and fix the easel base to the enlarger with that. A couple of inches on each of two sides would do the job.]

Just to state the obvious, don't forget that you lift the easel frame to slide at least two of the paper edges underneath. The paper will not stay so flat, or so still, if you just lay it on top of the easel.