There are some very elaborate ways to do this. I went with the quick and dirty. The door insert is made from a dump picked screen door. It was sawed in half and sheet metal screws connected the overlapped halves. Three layers of bubble wrap are duct taped in the center and a piece of corrugated cardboard (political sign stuff) was used for the velcro closing trap door. The last step will be foam on the sides for a better seal. There's 120' to the double stacked 6x6s.