Is your tub pretty tall? Getting one that is only 6 inches or so high, might fix the problem. 69-71 is on the low side, especially for a tub. I try to keep mine around 80, but it dips to 75 during the cold months in my 41 qt tubs.

What is the basking spot above the substrate-where your snake lays.

As for the mat only covering 1/4 of the tub, that's probably the main problem. The more area the mat covers the better it will be at keeping the ambient temps up.