"Keith and Wendy Slonecker" sez:
I would like to re-work my slate rock waterfall into one that has a large
flat top and the water "sheets" off in a solid wall of water into the pond.
Does anyone have an inexpensive or creative design idea on this? Ive seen
them in the pond shops made out of a sheet of pre-formed hard plastic but
dont like the way that looks exactly. I just would like to try the different
sound and profile that I've seen on some large scale ponds/fountains.

It's pretty easy. The trick is to start with roughly the right shape on the
rock (slate) edge, and then tidy up the edge with some liner or plastic,
just where the water is. Even if it extends a few inches past the end of
the rock in some places, nobody will ever know because they see the rock
and they see the water; the plastic/liner are underneath.

Liner does work for this, as long as it's just barely over the edge. For a
foot or so you could get away with probably 3cm with 45mil liner.