[FAUEE]

Your cabin shouldn't be wet, mdf "should" be fine.. Humidity isn't an issue with mdf. Actual water is though.

That said, plastic cutting boards at Walmart. We use them on boats because they're cheap hdpe plates. Not as easy to "build" with compared to mdf IMO, but great for Amp boards. A couple of pieces of abs or pvc pipe for risers and you can build an amp rack pretty easy.