Standard way to block sunlight from a non-rectangular window

My suggestion would be to pull out the window and clean everything really well to make sure nothing is gumming things up. Then I would use a clean rag and wipe all the areas where the window contacts the frame with a good quality silicone lubricant. A little goes a long way with silicone lubricant, you don't need to saturate things with a direct blast. I would avoid wd-40, or other cheap oil sprays, it could make things worse.

First, try to prevent the sun from hitting the building in the first place. Shade trees, large overhangs, louvers can all do this. There may be micro louver screens available that appear to be heavy duty insect screens but are actually very small louvers. You can easily see through the louvers, but they prevent the sun from hitting the glass.

Second, reduce the impact of what sun does hit the building by reflecting it away. Transparent reflective film on the window and light colored wall finish will work for this measure.

Third, reduce conducted heat. Insulate the wall so the hot outer surface cannot easily heat the inner surface. Use dual glazing the same way, the hot outer pane cannot easily heat the inner pane.

Fourth, prevent the hot inner surfaces from heating the room's air. Typically drapery or similar treatments. Not just the window, the wall too. Even sheer drapes you can see through are better than nothing. They trap air between the glass and the fabric, creating additional insulating value to keep the heat out of the room, and reduce the transmitted light that would have further heated the room interior.

I've covered how to reduce heat gain through the various modes of how it gets into the room, but I've only scratched the surface of ways to do this. Be creative. Figure out ways to achieve what I've outlined with materials that are readily available to you.