These are the joints for the FunForts kits (specifically this one). I tried printing this version, but not only were they a pain to print because of their shape, but the ended up being too big for the kit we had. I resized the ball and sliced it in two for faster, smoother printing.

The top-most hole is a little tight at first, but once you put the rod in and remove it a few times, it loosens up and gets easier. (And yeah, I now realize how that sounds...). And if you don't use a brim (I didn't) and get some curling/lifting/warping that causes them to not sit flush to each other, they still seem to work fine and hold the rods with a little gap.

Print 2 of these flat side down. I did 7% infill, no brim/raft, no supports. I used PLA, then glued them together with Loctite Super Glue , easy peasy.