This is an adapter for holding sandpaper with an oscillating multitool. I designed it for a DeWalt DCS356, but it may fit other brands/models with the same blade interface.
The Everclamp grid with sqwasher plate and thumbscrews lets you attach sandpaper, a sponge, or other irregularly-shaped object...
In other news, if you try to upload a file named uncalibrated_example.stl to Thingiverse, it fails with no error message, because somewhere they have a pr0n filter searching for rated*x.
Some people might find this bundle_scad.py code useful. It combines an STL, SCAD, and all its dependencies into a single file.
What material did you print this in? Sanding generates heat pretty easily and may soften some materials. I'm curious of your experience with that as you've given me some ideas for sanding jigs.