The bases are all used CDs, while the walls are made from foamboard (which I got free!) and covered with imitation corrugated sheeting made from packing boxes. "Lumber" was from an old basswood sheet I roughly snapped into pieces or from coffee shop stirrers (note: Starbucks and Caribou coffee have excellent wooden stirrers - much tougher than balsa/basswood, sized correctly for 25/28mm, and free). I tried adding some interesting bits and pieces to the groundwork as well.
Right now they lack windows, doors, roofing, and stovepipes but you get the general idea. The nice part is about this set is 75% of it can be used in various genres - Pulp, modern, or Post-Apocalypse settings.