A light duty square carabiner. Designed to hook to a bungee across my pickup bed to hold plastic shopping bags in place. This was designed to use a 7/16 x 1 1/4 (11 mm x 32 mm) compression spring or something close (smaller) but not so small it will fall through the squre clean-out hole. If you don't clean it up supports really good there is enough friction to keep it in place. A little acetone and a rag will polish the surface up nicely if you want the spring action.Keep the walls thick. You'll be bending the long side to accomadate the latch. Thin walls will crack a bit when using ABS. PLA may be fine. Heating them up may be a solution. I'll do some more playing when I get more springs and let you know the outcome.