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Have you considered the 2.5 D tile system - The DMG has great tutorials on how to build tiles from cardboard for next to no cost: https://www.youtube.com/playlist?list=PLw-XImXo4zfOcrcfnk1Rtg8xumInutBzyI put a set together with only a couple of hours work. There are great tutorials for doors, walls and traps etc. - a really nice system.CheersAlan
Pink sheets of insulation foam from any big hardware store and craft shop paint.
I bought a Killing Fields mat on Ebay, then discovered the guy who sells them is in the next town over LOL. It's pretty good quality overall, I was concerned about the fur length but 28mm figures stand on it fine. The color of mine is much greener and darker than the pics they have. Not sure if that's normal or just a one off. Like you I knew I could make one cheaper but also knew I never would.
There are great tutorials for doors, walls and traps etc. - a really nice system.CheersAlan