The reason racks don't come in a lot of mix and match sizes in one rack is due to heating. Different tubs require different amounts of heat and maintain their heat differently. So mixing multiple tub sizes in one rack makes it more difficult to maintain proper temperatures consistently for all of them on one thermostat.

I'm not sure how some of the home made creations accommodate this, or if they do...but that is the basic rationale.

The 32qt RBI racks with the optional dividers are a good alternative if you're looking for 16's and 32's.