I know just what you're describing, mine are the same way. My solution was to use the lids, on the 6q tubs, on the top shelf of my rack I have a short piece of heat tape closer to the front so I don't have to push the small tub all the way in to the back to get on the heat tape that is there for a 28q tub. The shorter heat tape is on a separate input on my Herpastat 4, this way when not in use I can disable that tape till I need to use the slot again for 6q tubs. If this makes no sense I can grab a picture later if you like?