We not long ago installed Roller blinds in a house that wanted to cover the whole recess!
the problem was, the window was 2.5m long, not short by any shame of the imaginations.
it was their daughters room, and she was having problems with the light and the sun hitting her and waking her up!
luckily, I had a popular fabric for this, and we went over the way we would install the roller, showed them were the blind would sit.
3 weeks later, I was back with the blind and fitting it for the customer.
well, when finished, even though still sunny outside, it was dark in the room, a complete success!
the Fabric was Unilux brand, I have used this in several of similar scenarios, biggest selling point about this brand is its cost effectiveness and the properties it covers for this price, you get a blackout blind, which is wipeable with a majority of chemicals, and looks good in different colours
the customer above had their in Red