Here is what I do for all my tanks My room temps hold at about 68f and my tank temps hold fine at about 79-81
First I cover three sides of the tanks with black construction paper or aquarium background
Then I cut pieces of cardboard to fit the 2 sides and the back of the tank
After that I cover one side of the cardboard with foil tape and tape it to the tank sides and back
This allows the foil tape and cardboard to reflect the heat back into the tank and looks nice as well
Looks like this when finished