If you want just removable, you could do a lexan window with Dzus or other quick release fasteners. If you want to do a hinged arrangement with a strut, you may need to reinforce the lexan window or it would probably be flimsy. Something like the GT Racing rear window frame (link below) and some 911R type decklid hinges might work.
911 Through 993 Rear Window Flush Frame Part# 51
Scott