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.