I make a recess in the middle watch section to take the back piece. I'm cutting down 2.0mm into the back of the middle section to leave a 1.7mm thick shelf inside the middle section. This gives strength and acts as a border between the top and bottom watch sections but I think it could be slimmer in future.
The middle section wall thickness is 0.5mm, so it's important to make sure the middle watch section is properly centered on the lathe before making this cut.