Do you need a pajama-friendly belt clip, but can only find mean-looking designs meant to be used with actual belts?
Then design a smooth 2D curve in Inkscape, and extrude it into 3D using OpenSCAD!
Just slice clip.stl for PLA, print it out, sand the edges to taste, and glue it to the object! (your call on permanence, but I recommend super-glue)
NOTE: The curve's stroke is exactly 0.8mm wide, so the slicer should not generate any infill regardless of settings (assuming your printer has a 0.4mm nozzle).
NOTE: The clip intentionally dips down just a little below its mounting-foot. This just means you'll need to use a tool to reach in and press down on the mounting-foot while the glue sets.
As seen in one of the attached pictures, it took a few tries to design a flexible clip. The first was rock-solid and thus useless, the second was hollow and brittle, and the third design just eliminated the cavity entirely and thus worked perfectly in both strength and durability. The forth iteration was me over-thinking the problem and adding surface area for the glue to adhere to, but I aborted on that for fear of gluing my tools to the phone.
This whole experiment highlighted to me how flexible yet strong PLA can be when you intentionally make shapes that are exactly 2-nozzle-widths wide, and I'll definitely be carrying that forward to future designs!