My sanding discs always ended up floating around loose in the bottom of a workbench drawer. To help keep my discs better organized, I made the wall-mounted storage unit you see above.
The case is just a plywood box made using rabbets and dadoes. The sides and middle divider have aligning dadoes that allow the trays to slip in and out for easy access to the sanding discs. Shallow grooves in the top, bottom, and sides hold a hardboard back. I also added a beveled cleat to hang the unit on the wall(the drawing below shows this). With the case done, all that’s left are the sanding disc trays. They are nothing more than short pieces of PVC pipe that are epoxied onto a square hardboard base, forming a small tray.
Cleats for Hanging