I used to keep all my sandpaper in a plastic bin, where I’d have to go through a bunch of sandpaper to find the grit I needed. I finally got around to making a simple organizer that I hung on the side of a shelf unit in my shop, where I keep my sanders. This can be easily wall mounted as well. One of the features is the use of whiteboard material to make it easy to reorganize and relabel what is in each slot.