I'm not sure the memorial is inverted - surely it's a representation of a cast of the stairwell (the hollow space), in which case the steps should be on the underside.
It was obviously not a literal cast (even if you could mould teak, you could only have taken a cast of the stairwell by temporarily filling it in)
I wouldn't be too worried about weathering - they used to build railway carriages out of teak, and they are generally left out in all weathers.