If your current heat pad is currently stuck on the bottom of your tank with adhesive, don't use it on your tub. Removing it from the glass may actually damage the heat elements inside and create hot spots.
Heating pads should be controlled by either a thermostat or a rheostat or dimmer, so that they do not overheat.
I use a 41 quart Sterilite tub from Walmart (about $8) with a locking lid. Get a solder wand and solder out ventilation holes (start with a few and add more if your humidity is too high).
I solder holes about an inch apart across the top rim of the tub and some in the lid.
You can make one nice hide for such a big girl with a kitty litter pan (small Van Ness also from Walmart). Solder out a doorway in the kitty litter pan with it upside down. Get a large crock from Walmart too for a water dish.
I place the water dish on the cool end and the hide next to it, so that the snake can thermoregulate inside the hide like this:
She's in the cool side of this hide:
I've used aspen and I currently use un-printed newsprint now for my substrate.
While you're at Walmart, pick up this digital thermometer/hygrometer in the hardware department with the indoor/outdoor thermometers (about $12)
I attach it to the inside of the tub with velcro - otherwise, the snake will always be turning it over. Run the probe to the inside of the warm side of the tub.
Good luck!