here is a quick drawing i did in about 30 min so some of the dimension might be off a little but it should work. you can adjust the window and window height per your liking
i used 1 1/2 x 1 1/2 framing for the blind and 2 x 4 for the base...with 1/2 plywood....some of the dimension will change if you use other materials.

i would just run a few 1 1/2 x 1 1/2 across the top and put some tin on it for a roof