I still remember when I was a little kid, my Parents got me a huge cardboard space ship, where I spent many hours, patrolling the universe and saving the day.
It is easiest to start with a large cardboard box such as a refrigerator or other large appliance. I called my local appliance store and had them save me a box. I told them I was making a cardboard playhouse and they were more than happy to set one aside. If you can't find one large enough you can certainly construct one out of smaller boxes, it will just take more time to piece it together.
