This is a remixed version of the micro sander. I made it roughly the same dimensions as the 5.5 inch Kwik sand hand sanding (so if you want you could use the adhesive sandpaper meant for those). I printed it in ASA with 4 perimeters, 7 solid bottom layers and 30% cubic infill (probably way overkill, but it's rigid and won't be breaking).

I just cut the sandpaper of choice to size and used some gorilla heavy duty spray adhesive and now I have a nice rigid sanding block for shaping rocket fins.