I want to build a shepherd wagon but (bowed roof)

Ok so I have been totally obsessed since the idea hit me to build my own travel trailer. Then I found this website and I can't get a good night's sleep all I do is think about it. Anyway, I have gone through several different designs on paper and in my head. I wanted to build the wanderer, but then I stumbled upon the shepherders wagon from a link I found on tnttt. http://www.schaeferwagen.de/oldtimer.html
Anyway, my question is how do I build the bowed roof like that.
(ie what is the easiest way) I need the inside to have maximum headroom as it will be a standie. The interior roofline will match the exterior in shape.
I already have my eye on a couple of pop-up trailers. And I already cleared out my last hobby and cleaned out the garage. God I hope I can sleep tonight.
Thanks for your help.
Anyway, my question is how do I build the bowed roof like that.

I already have my eye on a couple of pop-up trailers. And I already cleared out my last hobby and cleaned out the garage. God I hope I can sleep tonight.

Thanks for your help.