Welcome to MFK!!!
There is no way you're going to be able to insulate the tank to protect it against -5 degrees. Your best chance for success would be to construct a small lean-to style shed around the tank and insulate the heck out of the walls and ceiling. Then, use a small space heater, or duct the shed to your house heating to keep the shed temp at least in the 60s so you're not burning up aquarium heaters left and right.
Te shed only needs to be large enough to fit the tank. The wall at the front of the tank can be removeable or made as a swing-away door.
Good luck with the project.