Since I was in a hurry and I didn't have the right link for the site to link it properly, here it is again:
He seems to use screws like the ones that you mention, which I am sure are similar to what I used on the greenhouse.
Those are designed for metal roofs.
If I ever do that again I will try to find screws with larger washers.
Working with coroplast, I feel like a 2" washer wouldn't be too large--or screwing through a 1x2, like I had planned with 5 mil plastic.
Here is the other video that I watched:
I am sure he also used screws with washers. He sealed the back with aluminum tape and left it like that.
How would you have done it?