The core for Modern Pattern and Foundry was made from illustration board.

Having depleted my supply of large styrene sheet material, I decided to try something different for my structure cores, illustration board.  An email to a friend that has been using the board for years confirmed that it is viable and doesn’t warp over time as long as you brace it sufficiently.  A quick trip to Michaels and I was good to go.  It’s easy to cut with a utility knife.  For adhesive I use wood glue with the occasional spot of CA gel.  As further insurance against warping, I sprayed the finished core with shellac.