If I were you I'd throw some heat tape on the back wall as well as the sides of the rack towards the rear and put it on a thermostat at 90. Next is cover the sides, back and top of the rack with foam insulation. It's cheap. You can get a 4' X 8' sheet that's 3/4" thick for about $9 at home Depot. It's white with foil on one side. May not be the best looking way but it's low wattage and should give you pretty consistent ambient temps. Just an idea. ....