The cost of curtains for non-standard sized windows is insane too. I just bought canvas and I'm hand-stitching my own. All the materials needed for a dozen windows, including the tools, comes out to less than two curtains made by somebody else. As a bonus, you get to learn a neat skill that carries over to other things.

For reference:

https://www.theartfulsailor.com/catalogue/the-sailmakers-app...