strong sheet material is needed

Gday all,
My spa bath is about to be installed. The front wall under the bath about 500mm high will be built from bricks or from Hebel. It has to have space to accommodate the pump. The pump is not so small, and once we build that enclosure for it, it will stand out from the bath wall, like a box about 700x500, 500 high. I have little choice but tile over it. This box will be perceived by users of the bath as a seat, and someone may actually seat down on it. Therefore it has to be made strong enough.

What can I use for the top sheet of that box, under tiles, so that it may be tiled and used as a seat? It's spanning about 700x500. I can't place anything in the middle of the sheet as the pump will be there. The pump inside that box will be sitting quite high, so there is only 60mm available height for the sheet and the tiles (from the highest point of the pump to the top of the tiles). Structural plywood probably would be the best choice if it wasn’t wet area. I was thinking 2 layers of compressed fibro 18mm (total 36mm)? How strong is compressed fibro? (I have never seen it). Or fiberglass? Or some other wonder material?
Thanks for any advice!


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