I needed to sand my outdoor table and it has slats that are 4-6mm apart.  I'm sure there's a similar solution to this elsewhere but I didn't really look…  This is the design I came up with to quickly and easily get sand paper into those tight cracks while giving me a decent handle to hold.  It works amazingly.

The inner piece has ridges along it to stop the paper slipping.  The outer piece is under sized so it's a compression fit - no screws or clips to break.

Cut sandpaper to about 80-120mm wide (for the 3.6 x 20 versions) and to the length of the block.  Fold it around the inner piece - try to get it fairly tight around the end for a better fit but not critical.  Spread the outer (I use my thumb) and fit it around the end of the inner/sand paper combo by about 5-10mm.  Place the inner/sandpaper vertically on a flat surface with the outer around the top and press the outer down while holding the paper in place.  I find the flat surface helps keep the sandpaper straight.

Sizing:

I made this to suit my specific needs - 4-6mm gap with about 15-20mm depth.  If there's demand, I'll make some different sizes available.

Printing:

Print vertically.  I'd highly reccommend ASA/ABS (possibly PETG) for extra strength.

I printed using ASA with 0.2mm layers and 6 walls (the default ASA strength profile in Bambu Slicer).