Hello Claude,
for the commercial models each wall needs a mold. this mold is filled with the material that you have chosen (plaster or resin) and already this concluded the process of production. it is a basic and handmade process. Myself manufactures during many years my products, marketed under the label Blockhaus in a small garage.
They can become original with any material, it depends on the texture that you want to obtain, I have worked with plaster , cork, plastic, wood and cardboard with good results. Alone you have to have the caution in some of them of protecting them of the silicone of the molds with vaseline.
Charles , Bradley I do the roof with an old towel and painted with acrylic very diluted.
cheers
Carlos