In the sketch, and photos, I start with 25 1 X 4's and one 2 X 4. They all get drilled 3/16" edge to edge in 4 locations to accept 1/8" steel cable. When the boards are threaded onto the cables, a 5/8" rubber hose spacer is threaded between each board to provide a gap for ventilation. The bottom 2 X 4 is tenoned on the ends to match the thickness of the 1 X 4's, and the side guide grooves, the door will ride in. There is also a 3/4" cove routed in the bottom to accept a 3/4" steel bar. The bar has two purposes, to add weight to the bottom of the door and hopefully assist in a smooth closing and as an anchor for the steel cables that hold it all together. It is also bored 3/4" X 2" at the 4 cable locations, so the cable clips can store, when everything is pulled snug. When all done the bottom edge will be wrapped in a rubber shield for weather protection.
That will be the door construction. Hopefully it will be flexible enough to roll easily. I have a rough idea of the roll and drive mechanism. I have a couple of heavy duty worm drives I picked up cheaply, so one of them anyway.
My written explanation probably isn't much clearer than my sketch, but hopefully you can see where I am going with this.
Boards all cut to length and marked for drilling.
Set up for drilling. Its not perfect but it doesn't need to be perfect. A vise with deeper jaws would work better to compensate for the slight cupping in some boards.
2 X 4 routed and bored.
So I need to tenon the ends of the 2 X 4, then run some medium sand paper by hand to clean everything up and I can then apply the first coat of stain.