I have a chair that was missing a plastic end cap plug that went on the end of a metal tube, and I wanted to create a new one. The shape is simple enough, and easy to code up in openSCAD so it can be used with different tubing sizes. It took me a few tries to get a snug fit, but some trial and error is pretty painless if you have a 3d printer. You can use pla, flex, or really whatever material you want.