This is a mini sanding block that you can attach strips of sand paper to and allows for easier sanding with varying shapes (flat and round) for sanding of different shapes.

A fellow PrusaPrinters member (@TimTwix) noticed the flaw in the assembly process which requires you to force the screw through the hole and created a modified version of the main body and knob. They graciously allowed me to add their models to this post, so please consider their version for an easier assembly process. I've left the original file in place more as a reference than anything else. 

The original version requires an M3 nut and M3x16mm screw, along with a strip of sandpaper with your desired grit size (80,120,220,etc). Affiliate linked suitable products below:

• Sandpaper multipack
• M3 Screw pack

To assemble:

Place the knob into the main body slot. Then force the M3 bolt through the hole of the main body and into the knob. Then cut an appropriately sized strip of sandpaper (7" longx3/4" wide). Place the sandpaper onto the device as shown in the attached images. Then place the locking block onto the device and turn the knob to secure the sandpaper.