I personally dislike aquariums as cages because they don't hold heat very well. But since that's what you already have, what I would do would be to use just a heating pad/mat or heat tape on one side of the bottom. Put a section of foam insulation under the glass so that the heating element is sandwiched between the insulation and the bottom glass of the aquarium (this will help direct the heat up through the glass and keep the bottom insulated so it doesn't cool down so quickly) Position your thermostat probe either directly on top of the glass inside the aquarium underneath the substrate or on top of the heater under the glass so that it's sandwiched in between the bottom glass and the heating device. This way you'll know it can't get hot enough to burn your snake if they burrow down under the substrate. Set the temp to the mid to high 90's so that the temp on top of the substrate stays about 90 degrees. This should be all the heat you need. I also dislike bulbs for heat as I think they dry out the air too much and will cause bad sheds.