Because I'm considering several different clippable things for belts, I figured a basic, modular part for clipping things was in order.

This clip has a slot at the top which is continuous through its length, in addition to a deep cutout for the belt itself. This should make it easy to design things that slot into this gap for clip-on functionality. Click the thumbnail to the left for a better view with the default dimensions (assuming the export plugin worked correctly)

• Update: I've added a correctly-sized "plug" to the sketchup file and as an .stl. It should fit quite nicely, and could either be printed independently of another item and glued/welded on, or attached to a model before printing.

Check the dimensions against your intended belt; they should be pretty one-size fits all at present but it's best to be sure. Scale the plug to match!