If I may...
1. Futon
Every studio needs a futon because when working very late, your drummer's girlfriend can sleep on the futon instead of annoying the engineer asking studio questions. Though, watch out for face prints from excessive makeup. This may only apply to use with studios in New Jersey
2. Plexiglass over windows
Merceyful, its a good idea, and it works if you do two things.
A) get plexi at least 1/4" thick (3/8" better)
B) CHAULK!
The windows in my control room (garage loft) were installed in 1941, so you can imagine how much they suck in every aspect (though I like how the glass is somewhat wavy looking). Anyway, I chauked them shut inside and out until i get around to replacing them, then mounted above the windows curtain sliders, then for curtains I hung burlap curtains on the inside facing the window, and heavy wool facing the studio. You're welcome to choose less ugly fabrics, but i went for sound absorption.
The only drawback is my cat likes to sharpen her claws on the burlap
