Permanently enclosing the windows in a passenger van in an asthetically pleasing way is complicated because the walls are already nicely finished. The simplest way to achieve your desired outcome is with insulated window covers. They block out light completely and function as a thermal barrier. You can leave them up permanently or remove them easily as desired. Furthermore, it preserves extra width that would otherwise be lost if the windows were boarded up.
I've used Van Essential covers for almost two years. Prices of professionally made covers have come down considerably. They're still not cheap but I certainly couldn't DIY anything close to as nice. It's money well spent in my opinion.