Well, I live in snowy Colorado, but my 10gallon tank has a UTH on each side, each controlled by a separate thermostat, that keep the warm at 91 and the cool at 82. To keep ambient up, I use day and night lamps with 50 watt day and moonlight bulbs hanging so the base of the clamp lamp is about 12" off the surface of the screen lid.

Also, better than flimsy plastic wrap on the screen would be foil and duct tape. Three layers of foil, shiny side down topped and held on by a layer of duct. Keeps heat as well as humidity in. I also have the back and sides insulated.