Homemade waterfall spillway

To make a long story short, I have a small yard and therefore have a
small very upright structure built out of stone for my waterfall. It is
very square and I put a liner behind the face layer of stone (all the
stone are large rectangles) and underneath the top layer. Rodents ate
throw the liner in way to many places to patch. Oddly enough, they
didn't touch it for 3 years.

I am thinking to avoid this problem again, I should avoid a liner and
go with a plastic spillway. Since I need to retrofit it into my
structure, I don't think any of the pre-formed ones will fit properly.
Has anyone made a homemade one? I was thinking of one of those under
bed plastic boxes or something like that. Possibly using a much bigger
container like a huge rubbermaid that I can cut down to the right
height and keep a flap for the spillway. Any other ideas are
appreciated.