thats sucks mate. do you know off grizzly doghouses?
i have one myself and they have no entrance, they can be small because they have a door who goes both ways.
i would try one of those or something similar. http://www.youtube.com/watch?v=UxjG_ESdMe4

there are also heating mats for doghouses, grizzly can come with one too. maybe u have more luck with a different bedding in combination with a heating pad for the winter