I'm not certain I know what you mean by leaving the shelves short. If I understand correctly, I can see 2 possibilities with it. 1) - depending on how short you make it, it could provide a possible escape route. 2) - the extra circulation it would provide would just be a way for the natural "heat rises" affect to occur at a faster rate, causing your lower shelves to be significantly cooler than your higher ones.
Putting a back on the unit will provide some structural integrity, and help keep the heat in. It is not absolutely necessary, but unless the rack will be kept in a warm area, you will probably be much happier with the results as far as getting a good steady temp gradient if you put a back on it. If you are concerned about the weight of the rack, I suppose you could put something like wood paneling for the back panel. You would need to seal the wood.
I'd LOVE to know where to get 5/8 melamine for free!
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