I have the same problem.. I use a heat lamp with a ceramic heat emitting bulb and leave it on 24hours.. No light and keeps the tank between 80-90 no problem I have the bulb about 6 inches over my plastic tank on the same side as the uth..

Although I also have a space heater as my room is typically 65.. But the bulb can give u 10-20 deg increase in ambient just have to most occasionally as it dries the air