I made one similar to this. What I did was attach b to A. I made a whole long strip of 5" C fabric and a whole long 2 1/2" strip of D fabric. The sewed the D strip and C strip together and cut in 2 1/2" strips. That way, I didn't have to deal with that little 2 1/2 inch block. (Sorry, I did not take photos).
bkay