As for a screen, I don't know. I don't want to spend $500 on one and from the DIY I don't think I'd make it correctly. I may just go with the manual pulldown even though it is unreccomended.
I have had a manual screen, I have made a DIY screen, and I have bought fixed frame & electric screens.
The fixed frame screen that I made was easily comparible to the purchased fixed frame screens for quaity. The biggest downside was the time it took to make it. For me to make it for someone else would take 10 hours, which would end up being expensive for them, but if you are trying to save money, and get the best quality, then this is definitely the best way to go. I mean - DEFINITELY the best way to go.
Would it make a difference if I never retracted the screen and always left it down, would that prevent waves from forming?
No, this would not prevent waves from forming. Waves occur for normal environmental reasons. Heat, cold, and changes in humidity cause the waves to form. It's like your clothes in the washing machine - they just are going to get some wrinkles in them due to the process. Manual screens can be good for a year or so, but in the last 10 years of seeing manual and electric (non-tensioned) screens, I have never, ever, EVER seen one which did not have waves in the fabric.
There are multiple DIY methods available, so I would look at those AVS Forums a bit more and see if there is a version which suits your talents best.
I personally went with blackout cloth (BOC) and made the frame. Making the frame took about 2 hours. Stretching the BOC took about an hour. Selecting perfectly flat 1x3 wood planks took about 2 hours. But the final result was excellent.