This thing fits over an optical part holder. The original holder has a flat and slippery top which lets the optics slide off. This thing fits over the holder and is grooved down the center to prevent a lens, mirror, or other optical component from sliding off the holder.
This thing is in two parts and they are connected my 4 - 3mm screws. I made it two parts because it would have required support material to print as one part and my 3D printer is much faster and requires much less TLC when only one material is used.