Ok, the best way to heat any snake enclosure is with a UTH, or Under Tank Heater. Light from above is for lizards and other basking reptiles, and only sucks the moisture out of a BP tank.

Get a UTH or Flexwatt heat tape sized for your tank and a good thermostat. These will be the most expensive, but most needed, items for your snake. If you still need/want the bulb (for ambient temp boosts or viewing) make sure to raise it at least 8" above the top of the tank and use no higher than a 50w bulb.

Also, make sure the back and sides of the tank are covered and insulated and that your screen top is foil or plexiglass treated to keep humidity and heat in.