My mother in law is going through a similar issue in her house with her bearded dragons. She has the dragons in 125gallon and 40L glass enclosure and she couldn't hold the temps up in the cages, so all I did is went over there and wrapped the cages in foam insulation board and it brought the temps back up to where they are suppose to be. She took the boards down and painted them so they didn't look so ridiculous lol, but Maybe something like that would work for you. We live in Wisconsin btw and we are constantly combating negative temps here. Or heavy blankets if you have them laying around anything to help hold that heat in.