Probably good advice, but I should note that I’ve been in several home improvement stores the past few days and they all have green particle board. One even had green plywood. I’ve seen how conventional particle board becomes like a sponge to water, so it’s a bit surprizing that they can treat it well enough to make it suitable for moist environments.

I told a salesman that although I would prefer a foam cement board Wedi type of thing, it’s too expensive. He then suggested the green particle board. It looks like they soaked the sawdust so that every spec of wood gets the water-resistent treatment before pressing it together. So perhaps it would be good for this.

I’m currently leaning toward using sticks of green (treated) solid wood for the frame, and ordinary drywall for the surface because I have this on hand. Then I would smother the whole thing in a liquid rubber of sorts. The particular liquid rubber I would use is most common in roofing but they advertize that tiles can be glued to it.

(edit) The liquid rubber brocure lists things it can be applied to.

“To be applied on bituminous foundations and on the most common construction materials such as concrete, stone, wood, steel, aluminium, cement plaster and polyester”

Hmm… no mention of drywall. So perhaps I will need to make it entirely from wood before using the liquid rubber on it.