Keep in mind too, you will probably want to have the awning a bit wider than the window itself.
So you don't get that glaring sunlight through the morning and then in the arvo, and only have shade in the middle of the day.
Another reason to fill the ends just like GG mentioned.
The amount it sticks out/hangs down would need consideration too if you want light during the darker months.
I'm not a builder, ok, just some of the things I'd consider though.
Because once you've fitted it, there will be a reduction of light in the shed, and it'd be counter productive to run lighting in the daytime.
What material did the Wife have in mind for the curtains?
Was it Simpsons themed, Sponge Bob, flowers, or was it the colour?