I get Make, the newsletter from makermedia. In it was a tip on gluing foam sheets together. The source was listed as going around on forums, so why not ours?
Instead of glue, try Glidden’s Gripper, a Grey Primer/ Sealer. It is supposed to dry quicker than glues yet has great hold and works like the foam.
Foam can make a nice form for our mountains and other thicker structures…
John