What I've done to raise ambients in my plastic tub is heat cable along two of the sides. I have the zoo med repti heat cable 15 feet. I was told when I got this I didn't even need a thermostat and I haven't seen any evidence to the contrary. The cable itself gets hot and if you put it on the bottom of the tank it heats it up far too much but just along the sides it heats up the walls of the tank just a bit and raises ambients perfectly.