I actually made this as a threaded rod cap on my modified Mendel Prusa II with inverted-Z to verify that the two Z-axis screws remain in-step. However, I shaped them as the RepRap logo and added a keyring hole and have used them for several uses. They make a good advertising give-away or conversation piece. When put onto a 5/16" (M8) threaded rod, they are tight and the threads cut into the plastic (by design so they stay put)- I use a tap to make it quick. The RepRap logo is on both sides. I print .2mm layers in PLA without supports so there is no clean-up inside the hole.