My 1971 Sovereign
didn't have any blinds when I bought it and I improvised a quick solution that has worked so well I still have it 3 years later. I also used magnets and cut some thin plywood (wall panelling) to the shape of the window. The bottom of the wood rests on the locking latches for the window and magnets on either side hold the panel in place. While the wood panel is technically still straight it is so close to the window frame that very little light get's through. The magnets are from Lee Valley
and include cup to hold the magnet and a steel washer that screws to the wood panel. When not in use the panels fit in the back of one of the closets. I still have drapes around the front as the curved windows would make it hard to fit a similar solution.
If I was to do it again I would probably extend the panels to include the lower window as well but it was a relatively inexpensive and fast solution (~$40 and 1 afternoon to cover all the side windows).
Wood Panel - Note the small washers to connect with the magnet.
Panel in Place - One of the washer and cups for the other window is visible. I re-used the original mounting holes for the screen but in some cases they did not all line up and had to re-drill holes to ensure that the panels fit on all windows.