You have to be careful when using rubbermaid containers. Normally when they are used it is in some type of shelving unit that acts as a cover. If it's just a container you are using I would worry about the snake escaping and getting injured. You also don't want the humidity too high as that can cause skin problems.

As far as being too warm, a heating pad should be used with a thermostat, set it to the desired temp. Some people go as far as running 2 thermostats in case of a malfunction. setting the second one a tad higher than the first.

I would probably use a large aquarium and cover a portion of the lid with something solid. Plastic or plexiglass. That way you can adjust how much is covered to adjust the humidity.