I don’t know how big of an area you need, but the pond liner supported underneath at the sides by short lumber boards for walls may be the best containment
system. And sloping the bottom will improve drainage, though maybe at the expense of how much water you can capture. (You might look at
spill containment systems online and get some ideas.)
What sort of surface to the actors walk on? Is it a
deck over the containment area? Will they be
tracking water off
stage?
Draining that basin, I’d be tempted to find a way to pump it out, rather than put a drain pipe in it that has a joint that must be sealed. (On the other
hand, a pump won’t be able to pump it dry. Anyway to siphon it?
Trap door or below
stage access?)
Something to ponder – (sloped bottom not withstanding) will the basin be creating a big damp poorly ventilated area that can damage the floor anyway? I think you may need a sacrificial plywood floor under this.
Joe