Oldie but a goodie

lines on it for 1/2 and 1/4 sheets, or you can fold it.

Just screw the hack saw blade to the edge of your bench/bench leg in a convenient location and use the lines/fold lines to make the cut ... no need to make a base.

For some reason, neither method would work for me with either my worn-out or my newer sander; the quartered papers is always about 1/8" oversize making it hard to apply the hole punch on the width, and insert under the keepers on the length. So I have to fold mine into quarters, then just use an old pair of snips to cut off about 1/8" on two sides; with that 1/4" total removed they fit perfectly. I have a Makita palm sander - maybe it's metric. NBD though since I get a 4-fer with each sheet.

Twayne

I have a similar arrangement that I created about 35 years ago. It is inside a cupboard door where I store my sandpaper, I have multiple blades set to quarter a sheet or cut to fit a sander I have. Put a thin washer under the hacksaw blade when driving in the screw, the space makes it easier to slide the sheet under the blade without snagging.