Why i always recommend Ranco or Johnson for Metal racks as they are grounded. If flexwatt is wired up right youll be fine with just the thermostat. If you want to go the extra mile get the 6ft 3prong plugs, and do what was stated.

Black/white wires to be soldered to the metal strips, And green one can be Screwed to its shelf and then covered with Electrical Tape or liquid electrical tape.