Sometimes the tubs in a certain series are the same height; Sterilite tubs with the purple handles for example. the 6 quart, 15 quart, and the 28 are all within a quarter inch of each other. I'm kinda odd, but I use the lids with mine; your rack was probably designed to be used without lids. I'd try to find a smaller tub that is the same height as the 28's you are using, but that's just me. More hides would work as well as adding stuff to take up extra space. I use newspaper for a substrate, and if one of mine is acting stressed, I'll add balls of crumpled up paper to the tub for them to crawl around and hide under.


In this pic, I have 28 qt tubs in all but the second shelf. On the second shelf, they are 15 qt.