It's amazing that the enlarger requires such a deeply recessed board for an 80mm.
In any case, some flexible tubing that you can push onto the aperture ring when necessary would work, or possiibly shrink tubing used by electricians. In that case, you buy the closest diameter that fits, then it is shrunk to the exact size with heat.
If you have a good grain focuser, consider setting it at f/11 or so and leave it set.