You could put shade cloth over the plastic, or even build a second, smaller hoop within the first that you could have the shade cloth attached to (if you don't want your shading action to be so obvious to passerby who understand WHY you keep that shade schedule, lol), but your primary sheathing should be poly. The shade cloth is just meant for darkening, not for actually keeping the elements out.
It's not going to be a factor for my friend, who is WAY too paranoid to even CONSIDER growing pot! haha