>>thanks man. the bulb is a good foot and a half over the
>> tank though. would that be okay?
If the cage top is covered with something solid, the heat will not be able to get through (nor the UVB) although with a transparent top (like plexiglass or glass) some light will get through. This is not acceptable for the lizards.
If the tank top is covered by screen, the heat and light can get through, but the cage will dry out to unacceptable levels. Chronic dehydration is perhaps one of the most frequent causes of premature death in monitors. Just providing a water bowl is not sufficient.
Go to Home Depot or whatever other hardware store you happen to like. Get a cheap ceramic light fixture, a wiring box, a replacement cord, and a 1/4 inch acrylic sheet. Get a selection of halogen flood lights - say 25 watts, 35 watts, and 50 watts. get a thermometer (actually, you might as well order an infrared temp gun - I like to go through either the bean farm or Pro Exotics - google them for their web sites). Cut the acrylic to fit the cage top. Attach the light fixture to the acrylic on either the far left or right side, so a bulb screws in to the underside of the sheet, and the plug wire comes out on top. Drill a few ventilation holes (not many) in the acrylic sheet opposite the bulb on what will be the cool side. Set up the cage. Screw in the the 50 watt bulb. Wait for long enough for the temperature to stabilize (several hours). Measure the temperature. If it is too high, replace the 50 watt bulb with the next lower wattage. If it is too low, try with a higher wattage bulb. Repeat until you get suitable temperatures. If the temperature gradient is too uniform, you can drill a few more ventilation holes in the acrylic over the cool side.
Put in the work to get the heat and humidity right, and your lizards will grow up much healthier.
Good luck,
Luke