The drawings show U-shape plywood doublers both sides of each bulkhead where each stringer passes through. I did this to increase the gluing area between the stringers and bulkheads. This is the way that I built "Black Cat" and her structure has proven to be very strong despite the light weight.
I now feel that the doublers are probably not contributing much, if anything to the strength. Cutting, finishing and fitting the doublers is time-consuming work but they do make a neat finish. If you leave them out, they must be replaced by epoxy fillets to all three faces of the stringer. The fillets are messy work, so I will leave it to you to figure which detail suits your style better.
CONSTRUCTION PHOTOS (This section will expand to cover most aspects of the build)