At our old farm house, we had a large window like that. I made a roman shade window quilt.
I just quilted together a decorator fabric for the inside, with low loft quilt batting in the center, and white Muslim backing. If you don't quilt, you could even just tie it like you would a tied quilt.
Rolled the top of the window quilt over a thin board (to make it look nice and finished) and stapled it in place with a staple gun. The board will go inside (or if you prefer outside) the window casing and is held in place with brackets (sort of like you would a mini-blind).
Take down the blind after your sure it fits the window. Then use plastic round hooks (you can also use flat washers) and twine, and string it like you do for a roman shade. You could even actually use dowels like most roman shades call for if you wanted a really nice looking shade when it is raised. Our house was so drafty it was only up on really warm days. I took it down in the spring.
I put a tea hook in the wall to hold the twine when it's pulled up on sunny warm days (by just looping the twine around the hook); then you can just easily unloop it to let the shade down.
It was lightweight to pull up and down, but still dense enough to block out the cold.