For my bathroom/darkroom door, I found some draught-proofing strips that are black, and highly effective. If that hadn't been enough, I would have made a black curtain.(posted 9791 days ago)I've no experience with your enlarger. Some enlargers, especially those for different negative sizes, have an adjustment for _brightness_, rather than _focus_ in the corners. In this case, expose a sheet of paper with no negative in the carrier, aiming at a mid-gray. Develop it. If the corners of the paper are darker than the centre, there is too much light being projected at the corners.
I suspect that once the enlarger is set up for a given size of negative, it won't need adjusting.