Hi ~
A warm welcome to Geckos Unlimited!
Is your tank a 40 gallon breeder: 36 x 18 x 17 inches tall? Consider using 39 feet of heat cables configured like Kyle did. Heat cables will provide the most even coverage. Stick the cables underneath the base with aluminum tape.
If you choose to heat overhead via a CHE, I recommend a Fluker's 10 inch
diameter dome clamp-lamp (
not the deep dome). The All Living Things 150 watt CHE (hooked to Hydrofarm's Jump Start MTPRTC thermostat) fits that dome well and may work for you.
Some CHEs I tried touched the screen top and will burn it!