We saw a pantry cabinet (I thought KraftMaid) at Home Depot that had a magnetic steel dry erase board inside the door. I want to do the same. It was just a piece of enameled or powder coated sheet metal and therefore was very thin so it doesn't interfere with the rollouts. And being magnetic, a magnetic eraser/marker holder stays right on it.
Plan B is to cut a piece of sheet metal, debur the edges and drill the holes, and take it to be powder coated. I'm guessing if I can find a supplier for them, it will be cheaper to just buy one off the shelf.
I tried Google, but all I can find are 1,000,001 how to's on painting your doors with chalkboard paint or mounting melamine.