When I acquired a Club Wagon extended body with full glass and discovering no one made solid replacement window opening panels I concocted a number of possible permutations how I could DIY this in the driveway. For me it would have started with an accurate template that would account for the body curvature--got that solved at least in my mind.
The next challenge was how to attach the fabricated panels to the van body, wanted to use the factory type mounting and sealing system and came up with an idea that would have been nothing more than a test of concept.
Next up was what material to use Lexan or perhaps aluminum, the choice there dictating how I'd finish or paint the panels for a pleasing look, something that didn't scream out "high value items inside".
Sadly the need to put that vehicle on the road over ran the desire to create something I could not only use for myself but perhaps provide a "how to" of sorts. Lack of a space to work on a van with all glass removed didn't help much---I work too slowly when doing a project like this.
Anyway at this point I understand why the availability of these types panels don't exist or are rarely if ever seen out here in the bigger world.
Sorry for the detour--hope someone somewhere is making these even now.