RE: C-ARF Ultra Flash build Thread + Video
I think E is two pieces of wood. They may be the same size as the motor rails. It's been a long time since I built mine.
Edit. I laminated the two pieces E to the balsa block C and this was used to make spacers for the main gear doors and the nose gear door. On the nose gear door it keep the door from getting caught on the strut when dropping the gear.