Rigid board is probably the easiest way, put something like this around the rack/cage http://m.homedepot.com/p/Owens-Corni...-45W/100320352
It should hold the heat in better, you might have to adjust your hot spot, but you should have higher ambient temp. If you have too much air flow, it can carry away heat and humidity, but obviously you don't want too little airflow and have a wet moldy tub. If you still can't get the temps, you might have to cover the front of the rack/cage with insulation, but still need airflow to keep the tub from getting wet and moldy.
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