Hi Harry

Firstly welcome to the forum and how about a couple of pic's on your progress Im sure there are plenty of members who would be interested in your build.
Now onto your question, I too had the same idea about using the packing plastic for the glazing which should work well but I had trouble with the glazing fouling the sliding windows if I stuck it in the insides of the windows. Another way I tried was to cut an exact size to fit the window aperture but the glue made this look unsightly, the glue I used was Deluxe's Glue n Glaze by the way.The options I had were to glaze and lose the sliding windows or keep them without the glazing, I chose the latter. Hope this helps.

Regards

Andy