I got my foamboard sheets today! I got black ones, so I wouldn't have to paint them black first.
I changed my plan just slightly, in that I decided I wanted slight contours to the landscape. So this is what I've done today:
Four Foamboard Sheets
Draw contours for stepped rises in the landscape on thick cardboard
Start cutting the sections out and stacking them. Make sure that where foamboard sheets meet, the contours line up.
Contours in the corners of all four sheets
remove plastic protection from top side of each foamboard sheet
Glue underside of contours. Stick them on one at a time - and place the foamboard sheets corner to corner, so the different sections of contours line up when glued.
All glued! Putting them in the corners, the contours line up along both edges of the foamboard, so they can be rearranged to make different contoured landscapes!