A thought experiment is where you think about what would happen if you did something, but don't actually do it. It sounds like the story of my life. I will share my thoughts from yesterday as it may inspire someone else to do something.
My windows are a 76 inch drop (around 193cm). Did you know that the standard length of a duvet cover is 200cm (around 78 inches)? This is a lot closer to the drop of my window than the standard 72 inch, 81 inch or 90 inch curtains. The standard width of a single duvet is 135cm (around 53 inches) which would give a nice fullness if I used two single duvets converted into curtains for my 46 inch (around 117cm) wide window in the study. The standard width of a double duvet is 200cm (around 78 inches) and two of those converted into curtains would work nicely on my 90 inch (around 229cm) wide study window.
If I took, for example, two single duvet covers, which I could almost certainly get at a very reasonable price, all I would have to do would be to add heading tape and perhaps turn the bottom up with some weights in it to help it hang. All the seams would be done. And, so the theory goes, I could put a piece of plain white fleece on the inside as a lining. I'm not sure how that would work. It would make the curtains warmer, and I suspect it would block out a lot of light, but I'm not sure how easy it would be crawling around trying to get it to stay in place while you assemble it.
If and only if I managed it, I quite like the idea of having curtains like these (DH wouldn't enjoy them, but this is only a thought experiment).
And I would have the complimentary pillow cases to put over cushions. It wouldn't be much dearer than most curtains, and would be considerably cheaper than the current set which I am currently looking at with loathing. It may even be worth considering if you have less awkward window sizes but can't find the curtains you would like.
If I ever get beyond the thought experiment, I will share.