I had a Reimo that fitted under the open hatch of my Peugeot Partner. It was watertight - an elasticated band went round the lip on the car - which is a consideration. Not sure how the one Shawn found fits - it looks like there is no roof to the awning. The German text does NOT tell you much.
Actually, looking again at your first suggestion, Shawn, I was right in that it has no roof section. It fits inside the tailgate using press-studs. It is only suitable for hardboard-lined tailgates.
I think i'll track down some VWs with my tape measure and do a direct comparison of the dimensions!