don't go for a plastic one, go for stone resin. more robust but more involved to install. best way is to lay marine ply then bed the tray with cement. a bit of a faff but good and sturdy. any local plumbers merchant should be able to order one, there are a few manufacturers and they seem to be much of a muchness in terms of quality.
As Chris says, resin stone is what you want. Some have optional legs. Don't bother. Just use 18mm marine plywood. If the tray needs raising, stand the ply on 3" x 2" or 4" x 2" battens set 6" apart so the tray is fully supported.
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