Look for the metal film lined bubble wrap. One brand is called Reflectix. Cut a piece that you can lay over the top of the entire screen top. Doing this, in some cases a heat pad is enough and no lights are needed. In summer or during warm weather you can remove it.

I fill a tank about 4-5 inches deep with Reptile Prime coconut substrate. Then I tape or Velcro a heat pad (Mylar heat pads are the best, like ultratherm or flukers to the back wall outside the tank so that the bottom of the heat pad is about 1 inch lower than the substrate top. Moisten the substrate liberally. Lay a piece of 3/4 or 1 inch pvc pipe on the substrate butted up against the heat pad. The humidity in the pipe will be perfect. Tape a piece of reflectix across the entire back wall over the heat pad. Cut a hole for the thermostat probe.