This is a sanding block based off of a design I've seen before as a child. It's meant to have medium precision for crafting.

Print at about 20% infil, it doesn't need to be very dense, but it may need a good thick solid outer layer to handle any stress you put on it while sanding, I used a 1mm thick perimeter (about 5 solid layers with 3 perimeters at 0.2mm layer height).

It takes a strip of sandpaper about 1 inch wide and 9 inches long. Make a fold about 1/4 inch deep into one side and then double the fold to make the paper thicker on that end. Insert that end into the smaller slot on the block and then wrap the rests around until it reaches the larger slot. Use the wedge to wedge this end into the slot.

To use it, just rest it in the palm of your hand with your thumb on the wedge for leverage.