I used to own a condo built in the early ’70s. It had an 8′ sliding glass door in the living room and, in many ways, that was great. (I happened to be first floor, so moving the furniture in through a 4′ opening? Awesome.) However, as you might guess about anything built in the early ’70s, you would have to try really hard to make it any more energy inefficient. I could sit on the couch about 15 feet across the room (dining/living room was one big room) and feel the cold creeping across the floor to my feet. (And I live in Raleigh, NC! It still felt that cold.) As you curtain-buying/making folks might guess, curtains to cover an 8′ sliding glass door were not easy or affordable to come by — most sets are just short enough (made for a 6′ door) that I would have had to buy another set to cover the whole door.
I can’t remember if the idea came to me in advance or if I was wandering around Kmart and saw the Martha Stewart fleece blankets on sale, for like $20 every size. Best of all, they had red, which was my living room accent color. I bought two king-size blankets, sewed a pocket across the top, slid them onto the curtain rod and voila! And they made a HUGE difference in the comfort of the living room. They also looked pretty good and most visitors had no idea they were fleece blankets until I pointed it out.
The only downside was that they shrunk a teeny bit each time they were washed (once or twice a year) and by the time I moved out, they were not quite touching the floor, but considering that I got probably 6+ years of use out of $40 in fleece blankets, I can live with the shrinkage.
If you’re cursed with an energy inefficient window/sliding glass door, I encourage outside-the-box thinking — fleece blankets, vellux blankets, affordable comforters that tie in with your decor, etc. may be cheaper (and possibly work better) than insulated curtains.
For a nicer look, I’d suggest using a much wider blanket (or use two). Plus, if you’re using it to help boost insulation, the waves and thickness of the blanket(s) should help that performance.