The top voted item this month was updating the separate wall sets for cut-stone to the current standards. While I get that work done, I went for #2 on cut-stone: Secret Doors. With magnets, these secret doors are so secure and hard to detect that I generally have to work my way through all my walls twisting to find which one is the secret door. Your players will never pick them out ahead of time!
Grab the files on Thingiverse and let us know what you think in the comments!