I mentioned there were a few Montessori materials I made the night before Second Son was born. These rough and smooth boards took about fifteen minutes -- certainly a lot less time than we spent at the store buying the sandpaper. I modified the design a little from what I found in Teaching Montessori in the Home: The Pre-School Years because I couldn't find a fifth sample that wasn't black. (I put them on cardboard rather than the nicer balsa wood, but I wouldn't want to mar the looks with black sandpaper! Oy, I'm silly sometimes.)
These were ridiculously easy and practically free if you already have four kinds with graduated levels of coarseness. I just used glue sticks to adhere them to the cardboard.