Becky,
when I build melamine shelves, I try to leave no gap at the top. However, I do run my heat tape down the back of the rack. If you do want to put the heat tape on the shelf, I would recomend that you leave a very small gap, I would recomend about 1/32". That is just enough space for the tub to slide over the heat tape without binding. I used the same size tubs to build the rack for Joe that you can see in this forum a few posts down. It will have back heat on it though. As soon as I can get home, and wire up some heat tape, I'll post more pics of the finished product. Let me know if you have any more questions.