In an emergency scenario anything that water will not soak into and ideally that light can pass through will likely suffice well enough. Generally this would be a rain poncho or tent, not just big sheet of plastic.

You also don’t need plants and can do sea water or urine instead. Or, if you are in the desert, you will have to dig deep enough to draw from the moisture in the ground. But in both these cases it is important that the “plastic” funnel be transparent so sunlight can pass through.

https://en.m.wikipedia.org/wiki/Solar_still