Problem with "warping" of LEGO water I was trying to imitate the cool water technique of Legopard on MOCpages.

Sadly, I found that when I did it on a large scale (about 46x46 pegs), the water tends to warp upward in the transverse direction, curling the edges.  I cannot get my lake to sit flat.  I get the standing wave in the longitudinal direction, as shown in the picture, but the entire sheet warps in the transverse direction.
I first thought that perhaps mold type played a role, but building it in either clip type gave the same results.
Next, I tried putting in longer 1x plates below to hold the longitudinal wave as shown in the picture, while hopefully straightening the transverse curl, but the problem is that the 1x1 clips hold the water/tiles too tight in the transverse direction, thus pulling the pieces too close together, so the plates end up making the warp worse as they push the bases apart further (due to the normal tolerance or piece spacing).
I would love any ideas people can offer.  I am not sure how Legopard got his to sit so flat.  I am finding that even 6 tiles wide gets a noticeable warp and his picture is many times that.
This image of the same model shows that the whole set sits on a frame of bricks. In the inside, you could place bricks underneath wherever necessary to attach the water plates (not only 1x... but maybe 2x10, or several 2x4 in a row). They are invisible, so color and type don't matter. If this row of bricks reaches from one side of the frame to the other, it can shift to allow the longitudinal waves but it cannot curl up transversal(ly?).