Ok. I'm starting to get a better picture of what to look for. The plan is for it to fit tightly, so some sort of channelling on the sides makes sense. Curtains are an interesting thought... but I think steel is better for wind. More rigid.
I'm so new at this I hadn't even thought about whether they would roll down on the inside or outside of the opening. All the ones in the pictures seem to be inside, but just for room it might be worth building a sort of protrusion around the opening so that they would close outside.
Still hoping to hear from someone who has installed a few.
Like I said, I don't know the first thing about these doors. Just learning. In building anything, I like to start at the high end and then see what is being lost as the price goes down. For home projects like this, best quality interests me more than best price. After all, the labor is free. It's our home.
thanks,
rScotty