The side of a file cabinet (Black metal) and a few magnets would work! True up the file to match the enlarger and choose a suitable enlargement size it should weigh enough to keep it from moving accidentally. I've also found magnets that are the size of business cards (at Staples) that are covered on one side with an adhesive. Leave the protective cover on or stick some black paper on it. They are strong enough to hold down most papers, or cut them into strips to get more coverage! I also use them to hold down test strips when making prints on a regular metal easel. I get an unexposed paper white at the same time.