One triangular piece of cardboard that fits under the bed and is taken out at night to block the light! I wouldnt have windows like that in my bedroom, just living areas where no blinds are needed.

Cover a piece of cut- to - shape MDF with your chosen fabric or wallpaper . Attach a loop or tassel for easy removal . This can be simply stowed away during daytime hours .

Is there room for one or two 'flip-up' shutters in plywood, cut to triangular shape (one each side, hinged to sides, or single one hinged on bottom), raised and lowered by a simple cord, eyelet and cleat system? The whole window has an 'origami' look to it which makes one want to keep to crisp planes and avoid anything frilly. You could even have a different colour for each side...but, as I say, that would depend if the window is deep enough to accommodate the 'flaps' when open.

Thank you for all your great ideas!

Can your client be seen at night? Is that her concern?

Personally I love the clean unusual lines and wouldn't out anything up if I wasn't seen. If that isn't the case, why not have shutters erected and have square juxtaposed with a triangle? Or have them made to fit? Just a thought.

Our shaped Duette cellular blinds retract away very neatly and provide good darkening at night!