Secondary glazing on old crittall windows

[petercucumber]

Does anybody have a cheap method for getting these large, old, draughty crittall windows secondary glazed? There are 9 of these windows. I have bought 9 huge 4mm plexiglass sheets that are big enough to sit on the wall covering the whole thing. The windows are 1500x1200. I thought about using magnetic strips, or cutting the plexiglass into the shape of the window and using some kind of frame. The window opens at the top so this complicates things further. Another concern is condensation. Any suggestions/advice/ideas all welcome, cheers.