I'm not a doctor but I play one on TV. I haven't actually done this on my yurt, but that ain't gonna stop this google expert.
You might try tying a wood stiffener, like a 2x2 or length of wood closet rod, to the ropes below the bottom edge of the window cover, and THEN anchor the ropes. Drill three holes, pass the anchor ropes through, knot the stiffener to the bottom of window cover, and then anchor the ropes. That might work.
Here's how I solved a similar problem. For quick weekend camping trips I'm way too cheap to buy a camper. So, I made a gable roof tent frame for the back of my pickup, out of 1", 30 degree wall tent fittings and 1" emt. In height the gable ridge is about a foot taller then the cab. So it is not all that heavy I can handle setting it over the bed myself. I drop the 4 legs into the side rail holes, and cam strap it to the anchors Toyota conveniently located at the inside corners knowing I was gonna do this. lol
First go around at camp I worked the brand new 17 oz treated rectangular canvas tarp over the frame and drape the long edge down over the sides of the bed, and started lashing it down. No flapping at night for me, no sireee. 200' of junk Walmart rope gonna take care of that. An hour later my tarp looked like the goatskin head tied to an African drum. First night it was windy and the grommets snapped against the side of the bed and scratched the paint. That's what you get for being ..frugal.
Round two. I drilled holes in the face of a 10'- 2x4 and lashed it to the grommets on each long edge of the tarp. An anchor line at either end of the 2x holds it taut to the truck. VOILA! Easy to roll out and roll up. Super taut. No grommet noise no flapping, no paint scratches. Plus a good way to get looks tooling between campsites on the way to lake.
That's it. Good luck.