I want to do one on mine as well. From what I've understanding....the VW roof racks can be used but they are a narrower vehicle and the rack will need to have the mounting points altered. When you see how they are made....there really isn't anything to them and wouldn't take much to make them work.
Roof rack on a e-pup...Won't that look strange unless your bed is full?
I looked at old station wagon roof racks for vans when I first got mine. But went in a different direction. Of course those are too large for yours...make a vintage car trunk rack of some sort?
Here's a picture of a vintage Triumph Spitfire rack. Probably smaller than you want, but an idea.
I saw on thesamba.com a guy makes ol' school HWE roof racks. I emailed him and asked about them. He told me that he has made one for a Corvair van. He had them for the full length, half and for the bus trucks. Maybe he can make one for your Epup.