This thing replaces the clip that keeps a folded umbrella stroller folded. Ours was lost in luggage land.

The STL is for 10-11mm diameter frame rods, which seems pretty standard for North American umbrella strollers. Both sides snap tightly onto the rods, but I added a channel for a zip tie to prevent history repeating itself.

I banged it out in Sketchup, complete with manifold/watertightness errors; the STL is fixed in Netfabb Basic.