I designed a rugged sanding block using metal bolts and nice metal wingnuts (https://www.thingiverse.com/thing:3322026), but I wanted to have a similar block that would not need additional hardware in order to work. This is an all plastic version that should work straight off the printer:
Print a top, a bottom, and 2x knurled nuts.
n.b.This is absolutely not as durable as the metal hardware version, but it does work. To be really useful it should probably be printed in a stronger material like PETG. I printed the test version in PLA (because that was what my machine was set up for) and at low infill and it is relatively easy to break the bolts on the base. It can also be tricky to tighten and untighten the knurled nuts. Ive left it as a "WIP" because I may need to adjust the tolerance between the top and bottom: it is a pretty tight fit for thicker sandpaper.