I keep my balls in the "ball room racks" sold by boaphile which are 36" long. I would go with the 36" over the 24". Without a theromstat, it may get too warm on even the hot side with 2 feet of heating, but I don't think the cool side will be too warm. You may want to call Jeff directly and ask about his opinion, he's been building cages for a long time.

I've never bought anything from the other groups, somebody else will have to chime in on that one.