These clips are for attaching a burgee, signalling flag, or other item that you need to hoist up your mast, to a lightweight halyard to make it visible to others. They're not for heavy loads like sails or mooring lines.
I've seen these clips around and thought I could print them much cheaper than I could buy them! The clips are tied onto the ends of a line running up the mast (or flagpole) and slide together into the gap in each clip when held at 90 degrees to each other. Attach them to each end of the halyard using a halyard knot, or whatever you fancy, and attach another set to the rope running through your flag. Unclip halyard ends, clip to flag, run up flag. Simple.
I made two versions, one lightweight and the other much heavier but they are compatible. The hole without a gap is sized for 4mm cord.
No specific requirements, really, just a good strong print that won't break under the load of the flag(s) you typically fly..