you can get sheets designed for windows, its the same stuff buses use for the vandal shields, has a coating on it like a car windscreen to keep it together. just said perspex cos i couldnt remember the proper name for it, lol
i guess prob about 3 kg for undershield
have a look into getting carbonfibre doors made, the doors on a mk4 weigh a ton! seen a thread before of someone who did this to his, ill try and find it