Sanding tool 10mm width for slots. Also check out my other models and plce like them.

Print the parts. The handle horizontally on the print bed. Apply soft tape to the spot where the sandpaper or emery cloth will be applied (white in the picture). Cut sandpaper or linen 10 mm wide. Assemble the whole thing with M4 tread rod and nuts. Put a drip of (super)glue to fix the nuts in the insert openings. Good luck. This is a proven tool.