With the base piece to Optimus Prime's back box done, I carefully cut the two pieces of foam which would be the side. Things weren't a perfect fit as always, but that's what trimming after the fact is for. I slapped glue on the pieces and weighed them down with books.
When dry, I trimmed Optimus Prime's back box and glued it to the back of the torso.
After going for some coffee, my friend John stopped by to check out how the Halloween costume was going. He gave me a great idea for the grill. In order to make it look less flat sitting on the front of the torso, he recommended that I put two wedges underneath. I promptly cut two of them...
... and then did a dry fit:
Satisfied that it just might work, I repainted the grill (some pink was showing through) and painted the wedges.
Tomorrow I start the head. Stay tuned.