Placing the UTH on the side is very inefficient and allows for a lot of heat loss.
The best way to maintain proper temps during the winter on the cool side if your ambient temp is low is to have a second UTH on the cool side (underneath the tank) with a second thermostat.
You want to be careful with covering the tank, proper airflow is very important, what you can do is change the substrate to a coconut type bedding such as reptichip, and provide a larger water dish.