My son has one of those half moon shaped windows in his room. Custom blinds for these things are very expensive. The best option I found was something called a Redi Shade you can buy at Lowes or other places for 20 something dollars and cut to size, but it comes in basic white and needs to be stuck down with velcro. This window is better uncovered in winter so that light and warmth come into the room, but in summer the sun just beats in due to a western exposure. I wanted something I can remove in winter and replace in summer without dealing with adhesive velcro strips.
This window is 22 inches high and 46 inches wide so it's not a perfect semi-circle. And the arch is something like 6 ft so I knew I needed a long piece of paper to make a fan for it. Rolls of wall paper were not wide enough. I went to a teaching supply store and got a roll of 48 inch by 12 ft paper that is used to cover bulletin boards. This one has a cloud design, but they had many colors and patterns. This cost about $8.50. It also comes in different sized rolls both larger and smaller.
At a building supply store I bought two wooden slats 1 1/2 inches wide by 2 ft long for $1 each and painted them to match. These attach to the ends of the fan to weigh it down and hold it in place in the window. These were cut to just under 22 inches and I drilled a hole in one end for a ribbon to go through later.
The roll of paper is long and wants to roll back up so you need to weigh it down to work with it. I cut 5 inches off the width so that when folded in half it would be just under the 22 inches (height of the window).