The problem with a sheet of plastic is the media still get's in between it and the glass and you can't see and then the whole sheet of plastic get's blurred and then the whole thing needs to be replaced too often.

The least of the PIA is to tape up the window and remove what you need to see with a razor blade scraper. Then you have a nice little 3X3" window that is new to see through each time you remove a section of tape. Like I said eventually you will need new glass no matter how you do it. This method was suggested to me by the guys who sell the equipment and is the least PIA, I have been using my blast cabinet for over 25 years now.