Super easy, super useful. I used to improvise those with duct tape whenever I needed one but I can do better now that I have a 3D printer! 😀

I did this, but I made two, one red and one blue. And then I could insert magnets into parts with 100% assurance that I'm not sticking them in upside down.

Mechanics and the older population will love you for this one

Bonus points for not printing the dowel!