Here is how I have incubated Bearded Dragon eggs for the past couple years, and how I am incubating Ball Python eggs for the first time this year.

You need:

1 Styrofoam cooler
1 Submersible aquarium heater
1 Aquarium screen top
2 Bricks


Steps:

1. Place two bricks inside cooler and place screen top on top of them.
2. Fill the bottom of the cooler with a few inches of water (Do not fill over top of bricks).
3. Set the aquarium heater to 88-90 degrees.
4. Submerse aquarium heater in water in bottom of cooler.
5. Place egg-tub on top of screen top.
6. Put top on cooler.
7. Check temperature inside cooler/incubator in a few hours. Temperature setting of aquarium heater may have to be adjusted slightly according to ambient room temperature

I put a few holes in the sides of my coolers.

This set up creates a fairly stable incubator. My inside temp. sometimes fluctuates a degree or so due to changes in ambient room temperature, but nothing major. The heated water is what heats the cooler, which also creates a very humid incubator. I have hatched hundreds of healthy bearded dragons this way with no problems, and have had no problems so far this year with the ball python eggs.

Would post pictures but I don't have my camera right now. It is a very simple design.