maybe best to build each section to fit your boxes
(no gaps in between that way) example if you make a section 8 foot
and your boxes are only 30 inch x 3 sets = 90 inches thats 6 inch waste and means boxing in
so if you have 2 sections like that with 3 sets of boxes in each and a young bird section
by making the section fit the boxes means you get approx 1 foot extra in your y/bird section
hope this makes sense