Turn the storage tub upside down. With a marker, draw a door on the end of the tub. You can cut a template out of a newspaper to make it neater, then trace around it. Don't cut all the way through the rim that the lid snaps on; make your door bottom about an inch or two above it so it will be sturdier.
If you would like to add a curtain door, use a piece of heavy plastic or a piece of vinyl table cloth. Cut it out large enough to cover the door and leave it a couple of inches longer. Then take a piece of slender wood, like a ruler, part of a yardstick, or paint stirring paddles, and drill 3 holes above the door. Drill corresponding holes in your ruler. Place the ruler on the inside of the dog house with the plastic between the dog house and the wood strip. Bolt this down and you have your door.
If you would like to add a heat source, drill a hole in the back of the dog house large enough for the plug to fit through. You can add a heat pad. Just be sure if you use an electric heat pad that your dog isn't in a puppy stage where he might chew it up and get shocked.