My van had a custom interior (that I've since removed most of) that looked like it was installed by a shop.
The panels were thin boards, covered with foam and fabric, that seemed to have come from a kit of some sort because there were perforated cut outs in various places to clear things like power window switches and stuff.
In the back corners, they had some crazy shaped pieces that were screwed into the rear of the van, but were too long for that short area, so the installer had pushed the panel into an arc and then screwed it in on the van's long side too.
At the top, it was pretty much the same, but they didn't screw it in at the top.
It just kind of arced up and met with the headliner.
I don't know if that makes any sense, as it's pretty hard for me to describe, but they basically forced a thin sheet into the corner to form an arc, then screwed it into place.
I don't know how regular people do it, but that's at least one method that works.
It looked pretty good, too.
Unfortunately, I don't have any pictures of it.
I only removed that stuff because it was in poor condition.