I filed the negative carrier of my first enlarger to be larger than 36 x 24mm. This gave a black line around the image that was fashionable at the time. It can become rather hackneyed but works for some subjects. My early enlarging easels were run of the mill but very nice slate bed models were available with moveable borders, which allowed you to print almost anywhere on the paper. I used these professionally, and they permitted a 10 x 8" image in the centre of a 20 x 16" sheet of photographic paper if you so wished, without additional masking.