Clear PVC fabric as sound proofing?

I have just moved into a flat and there is a very large window in the bedroom that is old and not double glazed. How to sound proof it?

reminds me of the time we nailed an aluminium rail around the edge of a single pane glass window frame.
packed it full of rockwool, then secured a plasterboard sheet to the aluminium frame to seal it off.

Worked, just be careful not to pack it too tight if you go this route, as it expanded on one of the windows and actually broke the glass.