The easiest, most inexpensive and safest way to hold tubs with lids without "building" a rack is to use a book shelf...


After nailing the shelves in places so that the tubs barely slide in between them you wire up the heat tape, tape it to the shelves(not the tub!) and plug it into a thermostat.

Simple yet effective and tubs with lids hold in humidity as well or better than a rack system, the only hassle being dealing with lids...