I use a glass test tube for ABS juice (acetone/ABS) that I use to help prints stick, but the test tube rack is way over there [points across room], and that's a bit inconvenient. So I made a little holder for it that snaps the ~16mm diameter test tube into place right onto the frame of the Prusa Mendel -- or any other 5/16 inch or 8mm rod.
The clip alone can hold the test tube in place, but I didn't want the tube to vibrate out on long prints, so there's a base that keeps it in place.
If anyone is interested, I can clean up the code in a bit and get it online, too. It's parametric, so you can change a bunch of the properties to fit other sized equipment.
2) Snap onto 5/16" or 8mm rods.
3) Insert test tube.
4) Wring hands like an evil mad scientist.