It's cheaper like you said to simply get a sheet of polycarbonate, or if you want expensive lexan, plexiglass that sort of thing, cut it to shape and use rivets to mount it. Apply bathroom silicone before you rivet it on to get a seal. If you don't want to look at it, you can paint a strip to cover the rivert/silicone area
p.s Didn't know there are some vans that come with glass at the back. The glass passenger version is not like that, the windows can be slightly open hence why they are attached on 3 points and easy to remove.
I cant see a rear window costing that much from a windscreen supplier - just not autoglass as they'll pull your pants down.
I only paid 80 quid for a windscreen on my Bipper.
2015 1.6 L1850 90ps In Artic Steel inc Look Pack and Lighting Pack.