My idea was to make a magnetic sheet mask that would centre the image on the paper. (I'm using 35 mm, so there is otherwise one very wide margin.) Mask and paper would have the same outside dimensions, and in practice both would be placed on the (metal) easel together, after placing the easel in the right place on the enlarger table.

If no-one here has tried this and discovered unforeseen snags, I think I am going to give it a whirl. The magnetic sheet is cheap enough.