This I did and it is made out of a piece of heavy wall 1/2" tubing that I tapped out to 3/8 x 24 TPI on one side. A bolt, a couple of washers and the disc itself and it is done. I made my disc 6 inches in diameter to fit six inch PSA discs that I have a bunch of. Initially I thought about using plain wet or dry sandpaper but I could not get a big enough disc out of a sheet and still get a couple of discs cut out of a single sheet. I am going to cut a few more six inch discs out of a piece of scrap melamine I have here so I can leave a few different grits mounted and don't have to peel any off in order to change grits.
I made a trammel guide for my router to cut the discs out so it is easy enough to make them. I'll put a disc of PSA sandpaper on each side of each disc and write what grit it is on the side of the disc with an arrow pointing to the side the grit is on.