Looking at the Bovington Pz III at Tankfest yesterday, it began to dawn on me that the turret bin is assymetric ie the left and right side profiles are different. I think the photos below show what I mean. I'll take this into account when I come to make the box.
I just got out my dividers and measured off the Kiste in "Panzer Tracts 3-3" and as you said, it is assymetrical there too, though I had not noticed it either.