https://www.thingiverse.com/thing:586040

If you aren't satisfied with the ones you can find on amazon, here's one you can get a friend with a 3D printer to print up.

I just searched Amazon for “wire shelf connectors” and got lots of results. I’d check measurements, but here is one result that might work:

Dailydanny Wire Cube Plastic Connector 50Pcs Wire Grid Cube Organizer Connector for Modular Closet Storage Organizer and Wire Shelving (White) https://a.co/d/aQjeY8N

They're called C&C. I use them for making cages for my guinea pigs. There's lots of options on Amazon, they're somewhat universal.

Wow that's a blast from the past. I've still got one that's blue from when i was a kid. Sits in my attic torn down as they always fall apart if u bumped into it wrong.

Searching "wire shelf plastic connectors" on ebay will give you exactly what you want.

You can get them on Amazon. Look for wire racking an they will be in there somewhere.

Wow this brings back memories, my family had a bunch of these years ago I don't know what they're called but you could try 3d printing those caps

If you're crafty, I feel like this is something you could make out of a wooden 2x2 or a dowel stick, a jigsaw and a drill with the right bits.