trying to set up a new 10 gallon glass aquarium. got a UTH on one side with a 40W 'black light' heat bulb connected up to to a thermometer. top covered with plexiglass with about 6"X6" screen hole where the lamp is. bottom covered with about 3/4 inch moist shredded coco

the hot side levels at about 93 but the cold side wont get up over about 72.

whats the best way to get the 'cold' side hotter?