This sanding block was inspired by this design : https://www.thingiverse.com/thing:1515755 by Kent Wollesen.
I added one curve and a right angle, and changed the the way the paper is kept in place by using a spring.
The size of the block is 12x4x2cm (4.7x1.5x0.75 inches).
The size of the pieces of paper is 23x4cm (9x1.5 inches).
Technical
PLA (if you use another material, tou may have to change the spring thickness)
20% infill
Layers of 0.35 mm for the bottom, 0.5 for the top (to get a nice rounded corner), using Slic3r.