Rubbermaid Tote Cat House. Start by using a rubbermaid tote 18 gallon cutting a 6 diameter hole in the tote to act as an entrance. Two of the more popular styles are:

A large rubbermaid tote is a great choice. You can also purchase an igloo style cat house that has insulation and is better weather proofed. I use two rubbermaid totes, one inside the other, insulated all around, with a 6 hole in each so that the holes are lined up.