This sanding block grew out of a discussion on RCGroups after reviewing lots of the existing models on Thingiverse. This block uses metal hardware to hold the top down securely, and to add tension to the paper.
I used 2x these bolts:
and 2x these wingnuts:
But those are pretty fancy wingnuts and simpler ones (with no more than 1" diameter wings), with washers, would work too.
This requires a piece of paper that is just over 1/4 sheet.
You can print a pair of the hex plugs to cover the opening on the bottom after you have inserted the bolt. I glued these in with thin CA and then sanded the bottom a little to get it nice and flat.
I printed this, on an i3 clone, with PLA at .2 with 25% infill and it feels pretty solid in my hand with the metal hardware.
The RCGroups thread is here: https://www.rcgroups.com/forums/showthread.php?3129311-Tools-that-make-my-life-easier/page20#post40790999