For a sunny day around the 1100-1300 time-frame, I'd start with a piece of sky-blue posterboard, airbrush some clouds, and keep the vanishing-point behind the structures... Keep in mind that you'll have to curve the background from the sides as well.. You don't want the sides meeting the rear at a right angle... A nice, sweeping curve is best..
As for your viewing window, it depandes on how wide your base is... My rule of thumb is to keep it smaller than the width of the base by about two inches and square in shape... The front edge of the base should be below the window's bottom edge as well...
Once you get the base done and inserted into the box, you can experiment with viewing window-sizes until you get one that offers the maximum view without revealing any lighting, wires, or edges, or allowing the viewer to actually see the distance from the foreground model to background model at a side angle..
If my scene measured 12" x 12" at the front, I'd likely do the window at about 6 "x 4"... Building a reveal will help restrict the angle more, allowing your viewers to see only what you want them to see... But play around with the opening sizes first... Just tape some printer paper together to fit the front and cut different-sized openings until you get the opening size you want...
Just don't make a slit... Ever..
Also, plan for your lighting as well... You're going to need space under the dio base for the transformer and wiring junctions and plan some space up top too for the main lights and the mini spot & fill lights... If you're making a sunny day, or overcast, morning, evening, all that comes into play, as well as your main lighting, plus fill-lights to get rid of shadows in the wrong place.. If it's a sunny day or high overcast, you "sky" will be different colors, and your lighting will change as well...