Recently, I bought a pack of pre-cut 29x29cm boards made of 4mm thick cork, with adhesive on one side. Gluing them on heavy cardstock (240gsm), they turned into perfect terrain tiles. With four of them I can play games of Song of Blades, and Hordes of the Things in 15mm. Adding another five, I'll have enough for my Mutants and Death Ray Guns in 28mm. Here are some photos testing one tile with some ruin pieces. The first one features a crosspiece from one of Dave Graffam's sets I recently got from his sale on WargameVault.
This piece was made by wrapping cardboard with light gray paper (80gsm) and then coloring with pencils. After the glue had dried, I cut the hole on the wall and added some detail. The small obstacle was also scratch built, but ended being a little small for 28mm.