This. Exactly this. I don't have the rear bulkhead glued in on mine yet and it is easy enough to get the rear clip to lozenge from side to side. As soon as you glue the rear bulkhead in the clip, it sets that lozenge shape/rear clip width in place and then it makes it hard to adjust anything. I am waiting until i have my body all mounted up before i glue the rear bulkhead in the rear clip. Note that this is a construction process issue, and has nothing to do with the accuracy of the molds. You could clamp the bulkhead in one body and get it lined up spot on and then in the very next body clamp it in on an angle and have it all off and need to do lots of adjustment. Even though both bodies came out of the same molds.
Have a look at the thickness of the leading edge of the rear clip. its quite thick. Most of that could be an glue of some sort between the inner bulkhead panel and the outer part of the rear clip. That gives you some indication as to how much free space there is between the inner and outer panels.
Edit to add, doing great
@Tcollins
Ryan