Crack kills!

Back to that bulkhead diameter problem...  The black gap in the following photo is roughly 1/8" wide at its peak and that's WAY too large to bond well to the inside of the coupler:

All four of the provided bulkheads were this diameter and that requires a solution.  Last year I bought a plunge router and a pair of Jasper circle jigs based on recommendations from this Rocketry Planet article on cutting perfect circles and discs.  I clamped a sheet of 1/2" plywood down to my workbench and cut three new bulkheads just slightly larger than the inner diameter of the coupler (nothing a little sanding won't fix.

The front-most disc is the original provided by BSD and my three new discs are stacked behind it.  Problem solved.