I believe what you want to do is loft a solid between the two end formers. Cut the longeron slots into the solid and what ever other features you need.
You can then section the solid at whatever intervals you want. The end faces will then give you the bulkheads with all the longeron notches.
As an aside, I find parametric CAD programs less ideal for modeling purposes. The ACIS based programs such as KeyCreator allows you to work in wireframe, and convert to solids and reverse a little easier. Parametric programs are great for editing after the geometry is created.
Anything though is easier than trying to guess intermediate bulkhead shapes.
Hope this helps.