I got tired of shaking citadel paints by hand. This paint shaker does it at 500-2000 rpm via my handy drill. It's robust, minimalist and easy to assemble. No flimsy gears or easy-to-break parts.
It fits all sizes of Citadel paints, warpaints and any other paint bottle of similar shape and size. If not, then the holder can easily be adjusted in size.

It only needs four rubber bands and a little glue. Everything else is optional.


Tested with PLA and PETG.

Notes:

•  assemble once before gluing to see how everything fits together.
• 1,5 mm room all around the shaking pad to add padding. It reduces noise considerably, but is NOT necessary.
• Adding lubricant to the lifting area is useful if you plan on heavy use. Otherwise it's NOT necessary.
• 3 standard sized ball bearings (22mm) are recommended, but not strictly necessary. The holes ARE big enough. Push in with force while the ball bearings are aligned to press them in
• you CAN add weights to the 2 slits on the bottle holder to make it swing slightly better. I sized it for 2x10x20 mm magnets. Purely optional.

CAUTION: 

I tested my version at 2500 rpm for 1 minute. It held up fine, but I wouldn't recommend it. Also, it's not necessary to mix paint.
Lower and changing rpm work much better.