Interested in this too, did you ever sort something? From what I can see, the (insanely priced) Fiamma box just connects in with u bolts. Maybe you could look at something like this -
https://www.amazon.co.uk/Fischer-12...lpcontext&ref_=fplfs&smid=A3P5ROKL5A1OLE&th=1
I suppose it'd be down to the distance between the bars.
Ps. Is your carry bike fitted to a T6.1? I just fitted mine and it wasn't the easiest task. I've had to mount it really far left on the van door.