Get some long L joints and put them on either side of the bedroom wall, not inside the window box, then get custom made or home made drapes and use the clips the sell everywhere now to connect them to a wire system. They sell all these things on line. Clip them on the wire, slide open during the day and slide close at night. I don't know if this is feasible because it's hard to figure out the depths of the window.

Or a sliding door on overhead rods. They sell kits for that.

I was inclined towards the "sail" with a hook or button to hold it back. My problem is how to keep the sail top from falling down and attaching the rods to the wall. The keeping the top end up without falling down is a concept I haven't figured out yet.