3/11/2019 UPDATE: Ultimately I wound up putting a small circle of leather in the bottom "cup" of the holder. That seems to both tighten up the grip, and provide a bit of a cushion. I've been using the CatShaker for a lot of painting recently and it's working really well.

2/23/2019 UPDATE: I've discovered that the action is so vigorous that the top of the paint bottles actually start to eat up the "forks" at the top of the holder. I'm experimenting with gluing small strips of leather in place to see if it provides enough buffer. I'll report back if I make any changes. I may need to re-design it as more of a clamp rather than a static retainer.

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Anyone who works with the dropper bottle type paints knows how vigorously you have to shake them and as I get older my wrists just won’t put up with it anymore. I saw Jollybgood’s design https://www.thingiverse.com/thing:2684065 and liked it but I don’t have a reciprocating saw so I started looking around my shop to see what I did have available which is a Ryobi “Corner Cat” finish sander.
https://www.homedepot.com/p/RYOBI-18-Volt-ONE-Corner-Cat-Finish-Sander-Tool-Only-P401/206354299

I borrowed the bottle holder from Jollybgood's design, designed a mounting plate and put it on the sander. I also put a single BB inside each paint bottle to act as an additional agitator and it only takes about 30 seconds to seriously mix the paint. I held it so the bottle was horizontal and rocked it slightly back and forth while it was running.

The best part is that assembly is as simple as removing the 5 screws from the hook and loop sander plate and pulling off the plate and then screwing the paint shaker in place. That’s it and you can go back to using it as a sander any time you wish.