That's really frustrating to find out that the dimension of the parts are so far out that you have to figure out how to modify them to fit, especially when you are paying premium prices to find that out. Instead of tearing your already-assembled turret apart and trying to narrow the parts before reassembly; maybe it's worth contacting Christian and letting him know what you're running into. I would hope that he would want to improve the quality of his kits, and maybe feedback from builders is what helps him do this. If he were willing to recalculate the dimension on the turret parts sheet, that would fix the problem for any future sales of the kit - and maybe he'd even send you a new turret sheet as a "finders fee" for having steered him in the right direction. Doesn't hurt to ask...
-Mike