I scanned the site very quickly and only saw one box that was about the same size as the sweater boxes you are looking at. I believe they called it a boot box.
I did not see a half-sized box which would be helpful for the type of rack you are building.
At any rate, I have seen these boxes in person. They do have feet on the bottom but they are much more sturdily built than the sterilites that your original post was about.
I believe these boxes won't sag around the feet and will remain escape proof. However, the feet still prevent the boxes from sliding efficiently in and out of a rack. Not a huge deal but one to consider nonetheless.
I was in a Bed Bath and Beyond tonight and saw that Iris does have a sweater box in the 28 qt. range. I did not get to take a close look as the GF was in home shopping mode if you know what I mean.
Worth checking out. Reptiletubs.com carries a full line of Iris' products.