This is a remix of Dierks36's model from Thingiverse.  I removed the center pegs to allow for sandpaper without holes.  It works well for the normal discs I have.  3M Cubitron Net discs are a little too big.  I imagine you could size the model up by a percent or two and they would fit perfect.

I also included a model with no pegs at all in case you want to put them in a drawer with no lid.