I added some French signs on to a board that I made from card & thin plastic. And cut out a piece of plastic roofing tiles, I`m going to try and finish the windows with broken pieces of clear plastic and some with balsa strip wood & matchsticks. Yes you are correct I used" spackle" and sealed it with pva . I also use" hard as nails".

The basic structure is foam board.

Spackling is used to fill cracks and round the corners.

Elmer's Acid-Free Foam Boards, 20 x 30 Inches, 3/16-Inch Thick, Bright White, 2-Count (902015)

If you are looking for high quality, archive quality or you are looking for a backing for photographs you should use an acid free foam board It is more expensive per sheet though.

Lightweight Spackling