This one is mine - shown on my old E83, the principle is the same.
The box is mounted so that when the tailgate is open there's 20mm clearance between the back of the box and the spoiler. The front over hang isn't an issue from either weight or vision. It's a Thule 700 Evolution approx 2.25m long fitted to BMW rails with the Thule T-Bar fitting system which I highly recommend to cut down wind noise.