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.