just weird, I had no problems heating a 41 quart with a foot of 11 inch flex, my room was about 72ish most of the time... you got alot of air flow pulling heat away? big gap in the top of your rack and the tub? or... im trying to think what the other variable would be causing the huge difference. something seems wrong. seems like your room temp is much lower than 70. got a link to the rack design?

i've always used newspaper, another substrate might make it colder where the snake sits.