Plastic should hold up. I've seen hoods made out of everything from wood furring strip frame with poly sheet covering (4 or 6 mil), to aluminum sheet. Some made of rolled roof flashing, others made from sheets of aluminum gauge unknown, to ones made using SS. They all had 1 thing in common, they moved the steam up and out.
Which you choose depends on your budget, and some of it depends on your ability along with how long you expect to have the evaporator you are making it for. The choice is yours alone.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.