There is no room for panels on the single window.

you need at least two times the window width.

you want them to look full when they are closed.

for the single, extend the rod to the left of the window and push them all to the left side, to clear the window trim.

Any chance you could show me a photo of what you mean @lyn nielson

I am sorry, I meant to say to the right of the trim... kind of like this, the best example I could find. only with the rod extended beyond the trim, so you don't conseal the window opening.

These windows are really tricky for curtains. They need to be skinny curtains because on the double window, the radiator is almost next to the left window so not much room to stack when open without covering the window. The single window yes, needs to stack only on the right side. The fabric is great and perfect for your room. I just wonder if curtains are right for these windows.

IMO drapes are either full enough to look good even when closed or no drapes , fake panels are IMO always bad . Your window is lovely , privacy does not seem to be an issue so I would leave it as it is , It would be nice to see the whole room but IMO those drapes will drive every single choice you make for anything you add to the space .THye are already a conflict IMO with the art.

If you are super smitten with the fabric (which I am!) you could do an inside mount rod and make them into cafe curtains that stop at the sill. You could make them single panel that always stays open or two panel that can push open, though they make looks skimpy that way. Here are examples of both.

Cape Style Excellence · More Info

Dining Area · More Info

Gee I don't see how you think this fabric works in your room. See example below. Colors are all wrong.

Because of your window positions I think some type of shade is your only recourse except perhaps shutters.

https://www.smithandnoble.com/shades/roman-shades

The window well isn't deep enough for inside mount anything. Can't attach a rod bracket inside of the window track.