I also have a few BARRs cages. I have my 20x20x20 (which houses a green tree python) set up with rope lights run in the inside (around the ceiling and in front around the door) to provide indirect lighting. It seems to work very well since the rope lights I use are indoor/outdoor and so are weatherproof. The lights provide some warmth as well but don't get burning hot. The ropelight enters the back of the cage through a 1/2" hole (slightly larger than the diameter of the rope light itself).
The way I attached them was to drill a series of small holes in the ceiling of the cage and thread tie-wraps through the holes to hold the ropelight.
The ropelights work with a dimmer, too
