Ran across this during a search and thought I'd offer some updated information. I hit up BoxManufaktur directly through Facebook and they were extremely responsive, asked to give me a call to discuss details right away. I can confirm they only do production in Germany but have a container coming to the US each month so can ship either kit or finished product in a reasonable window. For those looking for a source, according to their site, they also do a number of Euro based window solutions that folks sometimes have trouble sourcing such as Seitz Dometic, Mekuwa, Outbound and KCT. Also interesting is they do DIY supply as well. So they'll provide as much or little as you need rather than just a couple set models.
I'll be connecting with them soon to just kick the tires on some pricing. They do everything custom to spec, vehicle, etc so they didn't have a price list that I can pass on.
Hope this helps. Cheers!