Invest in a good thermostat. A Herpstat Basic is available for $115 USD + shipping and is perfect for a rack, but as long as your thermostat is proportional and not on/off, it doesn't really matter what brand. A proportional thermostat if set to, say, 90 degrees, will always heat the tape until the thermostat probe 'says' 90 degrees. An on/off thermostat, if set to 90 degrees will fall and rise, fluctuating a few degrees either way. So instead of staying steady 90 degrees it'll fall down to 88 before the tape gets turned back on full blask, heat up to say 92, then turn fully off again. This is much more rough on your heat tape than a proportional, which controls the amount of power getting into the tape, therefore controlling the heat that way.
EDIT: Well, I shouldn't say that you shouldn't use an on/off thermostat at all, just that most people here prefer and recommend proportionals.
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