Low-cost lab rotator

I'm doing a lot of magnetic bead DNA purification protocols, and they frequently have a step that requires rotating the tube at a low speed for 5-10 minutes. Until now, I'd been taping the tubes to the rotator shaft of the forgotten hyb oven in the hood of the lab next door. Then I realized I had a set of spare parts laying around that could do the job for me!

This lab rotator is designed for light duty around the same DC gear motor and PWM board I used in [my low-cost remix](https://www.thingiverse.com/thing:4143855) of the [Adafruit Crickit lab shaker](https://www.thingiverse.com/thing:3072610). It uses a USB charger as a power source, and two ball bearings to keep the motor shaft captive. I designed it with a modular, magnetically-attached tube holder so that I could swap it out for different tube sizes in the future. There are spaces for eight magnets, but in practice a pair of [1/8" x 3/8" magnets](https://www.kjmagnetics.com/proddetail.asp?prod=D62) or four [1/16" x 3/8" magne