Originally Posted by dubmunkey
Im not sure plexiglass is any more soundproof than mdf... But im sure there is a material you can use that will be better, could be pricey though...
I got very good results insulating a wall in my rental flat towards the neighbour using this: (sorry, the page is german only, but the images behind the "Fotogalerie ansehen" links might help understanding the principle): PHONESTAR Schallschutzplatten Wolf Professional, Twin und Tristar
It's layerd corrugated cardboard, the hollow spaces filled with glass sand which makes them very heavy and dense.
It's very effective keeping out everything above low bass frequencies and better than everything else (including mdf) I tried. Maybe you'll be able to find something similar in your region and use a combination of MDF and something like this for your windows.