...the bottom of the cage UNDER the glass, NOT inside the cage. Place it under one end butted up to a corner. No you don't need to actually attach it to the tank bottom. Without a dimmer or thermostat sometimes it's best NOT to attach it, but rather have a space BETWEEN the heat pad and bottom of cage, as a space will allow excess heat to dissipate before rising to warm the glass. Lay a good working thermometer on TOP of the glass right in the middle of where the heat pad is. Better yet, put 2 there. CHECK your temps, don't guess, so that you get about 95F. Also don't use too big a heat pad. As a rule, about 5-10% of the floor should be heated at the END of the cage to maximize your thermal gradient. Most of us "serious" BP keepers use a product called "FLEXWATT", which is a line of low wattage heating material ideal for belly heating. Look for it in the classifieds. We also check our temps with "temp guns" (infrared sensors) which have become quite affordable, and digital thermometers, with permanently-placed probes.
By providing belly heat, you can greatly reduce the wattage or even completely eliminate the dome fixture over the cage. THEN, cover the screen lid with plastic sheeting or plexiglass, but leave a space about 1" wide at the cool end of the cage. It may help to TAPE the plastic all the way around the perimeter (except the 1" space). This will provide a Vapor BARRIER which will help trap in humidity, which may be increased by spraying with room temp/lukewarm water.
Goodluck!
Scott J. Michaels DVM
Serpent City