100MM EPS foam painted and glued onto the back and 2 sides with sheets for the top and run water proof led lights under the foam, Make sure they are water proof!!
At night place 1 more sheet on the viewing panel to hold the heat in and paint this black. your temperatures wont drop at all. I tested this method on a 2ft tank to prep for my turtle tank which is outside. The 2FT held its heat over the night without the heater in the tank. During the day I lost a bit because I cant use LED lights for turtles so the top and front have been open but its holding quite well.
if you want a nicer looking system =P build a stud stand/wall and pack it with EPS then stain the outside and have the view panel showing then cover it with a sleeping bag at night.
Or spend countless $ on heating air to heat your water