@ a new DIY-er: I found a company that makes moisture-resistant MDF, and even a moisture-proof MDF for outside applications. The company is Nantucket BeadBoard. The MDF is green like greenboard, is primed, and is soooooo smooth. I'm going to see what happens if I put a piece of what they sent me standing up in a pan of water.
I am still considering just having it made for me, though I will need to have the knife made first because I couldn't find anyone with the wider pattern I want, but having the knife made isn't really very expensive in the scheme of things.