By popular demand, here is my window curtain bodge . It does involve tape, but not duct tape: because of the thin and clear materials, I used clear packing tape.

The photo below shows the wrapping paper shades pulled down to keep the sun out. They are taped to the wooden window frame.





The photo below shows the shades pulled up. At the bottom center corner of each I taped a band with the sticky side facing out so that it sticks to the wall in the up position. I also put a bit of tape directly on the other side so that when the tape curls over in the down position, it can be easily lifted off.



I could improve the bodge by making a sharp crease on the diagonal so the shades can be correctly positioned against the wall without any guess work. Notice right side: nice, left side, not nice.






The photo below shows the bubble wrap I'm using (recycled from some parcel I received) to let the sun shine in while keeping curious eyes out. Each curtain is taped directly to the glass with clear packing tape:






Voilà!



If you've ever tried to buy a "window treatment" and found the choices too fussy, too polyester-y or too pricey, you may enjoy a shortcut like this and mentally send all the curtain-sellers to hell.