This design was uploaded to Thingiverse by user acadey. The original entry can be found here. 

Thingiverse description:

"This is an 8 tube magnetic rack for 1.5mL tubes. These things are insanely overpriced and are a perfect thing to 3D print - the major cost is the magnets which are ~70 cents a piece. This design has little arm things that hold your tubes in place snugly and can even hold them in when upside down. I recommend using these magnets:http://www.kjmagnetics.com/proddetail.asp?prod=B842and using some epoxy to glue the magnets in. The easiest way to glue them in is to glue an outer one first, then put a tube in to hold it in place, then do the same to the third position tube, then the second one can be glued in since it will have equal magnetism to the left and right. Repeat for the rest of the rack. This design is optimized to use less plastic and prevent warping during printing, though I still recommend printing a raft just so it gets started easier.

I updated this model with a different base (it now has more room for you to look at the beads when pipetting) and also a version where the tube grabbers are printed without an overhang as per suggestion in comments.

Also if you are a grad student and are trying to convince your PI to buy a 3D printer - making just a few of these more than pays for the printer itself!"