Its simple. They order some parts from Acme... Some were pretty big. One box containing some parts was the size of the VAB... Actually its the VBA now! Was colorful inside and out with pictures from Earth and the Acme Anvil Logo on the side. Since the Kerbals like the box. And were a bit low on cash and paint. They reused the box as there VBA! But could only decorate all but one wall inside... That "Outdoor" wall gave an nice air and calmness without needing to afford the cost of glass that big! To this day. The Kerbals have no clue why earth has cotton balls that big floating around in their air! The doors are the old flaps on the box. If you look real close you may see a Acme Anvil Logo and a Made on Earth mark where they just ran out of paint.