So when you roll the little veg plants into the flower units sometimes the fan wants to flip them out of the 2" hole in the post. Duct tape was the first solution... this is more permanent: $7.50 / 100 + that much in freight.
http://www.cabinetparts.com/c/mirror-and-glass-clips/SPHU26000/
[Crappy phone pic, but you get the gist]
Use # 6 x 1/2" stainless steel screws..... and I installed the tabs upside down because they have a short tab on them. If you want to file down a mess of them you can install them right side up. same diff really.
Use a tiny drill to make a pilot hole using the tab as a guide... don't want the round part covering the 2" hole. Don't screw them down too tight or you can't spin it around.
Enjoy !