So I went out in the pouring rain the other day - jumped up and down on one of the rocks to settle it a little bit more than the others.
I got wet but the water feature got levelled. I used the water in the container as its own level.
It isn't perfect but then again, neither am I. So we compromised at almost perfect (as I am) :-) and given it is plastic, I think it is as close as its going to be.
Moral of story. Sometimes not quite perfect is really good enough.