Yes, I know exactly what kind of window you mean – very 50′s indeed. I think your best bet is to go for clean lines, maybe with a little retro modern look. My first thought is a fancy/decorative roller shade. The Shade Store has some great options with blackout shades (if you really want to get it dark) or light filtering shades, which come is some fun patterns.

This one is super cute, though the patter may be too big for thin ribbon windows:
http://www.theshadestore.com/template/images/portfolio2/large/145_l.jpg

But, you can check out their full collection here: http://www.theshadestore.com/product2/roller-shade-light-filtering

The only other thing I can imagine is maybe a roman shade? You can google and find all sorts of Roman shades, but here are some ideas: http://www.selectblinds.com/romanshades/select-fusion-elegant-collection-shades.html

If you go with shades of any sort, obviously they will bunch up on top, instead of 6 feet of material on the sides. I think you could also split the window – meaning install 2 36″ inch shades, rather than one huge 6′ shade. That would also give you more light filtering options.

It’s hard to get windows just right, but I always find it’s worth the effort and worth the money to get a proper light filtering solution.

Good luck! Send us a picture if you come up with a good solution.

Thanks!

Love the idea of using ribbon – consistent tab width, no edges to finish, and a little sturdier than my fabric.