If mechanical agitation is what you're after (instead of heat) then an end-over-end rotator for 50 ml conical tubes is what would normally be used in a lab, and what would work best for this purpose. A basic one would run about $200, and you wouldn't have any other use for it, so your best bet to achieve the same thing, that you might already have laying around the house, is a rotisserie cooker for ~$60. Strap your bottles in there, set that sucker to no-heat rotation and you're in business.