Most every build, I try a new technique or two. Sometimes, its an innovation, other times, it represents "doing it the hard way". This time, it was bulkheads cut from 3" minicel foam:
These were much easier than I expected. The compress-ability of the foam gives some leeway-you can simply push the bulkhead in until it fits. There are some gaps, but I think I filled them adequately with caulk. Much easier than whittling a piece of plywood until it fits, big fillet of dookie, etc.
Side brace plate visible on the left.