The sides of the box go all the way back to the front of the canopy opening. They have tri stock on that former as well as small braces on the box sides that tie it to the formers inside the fuse. I guess if you use a larger firewall you could tie it back to the front former wit some thin ply and tristock but its not really needed.

Also if you need 4 inches to mount your engine, cut the box sides off flush and you should have 4 inches. I overlapped my sides and added the tristock for added strength. just a thought..