Most people have a dull belt sander belt laying around the shop. I use them to make sanding blocks. For a 3 x 21 belt cut a piece of wood 3″ x 19 7/8″, I like MDF, because it is hard and stable, plywood works good also. I round the corners on the ends of mine, but I have some that I use a 22* angle on one end. after you get the wood cut just slip the belt over the wood and presto instanced sanding block. The dull belts last a long time and work great for hand work even though there to dull for the belt sander.