To start with, doing a box with perpendicular walls like it was before is complicated and prone to leaking, but I was going to stay on that bent.
When you cut the large irregular opening, I had to switch to plan B, then plan C, and so on. I finally settled on Plan R.
Cut the OSB to the 8" by lenght of the cut, + an inch. Lay it against the box and you see the light. It won't fit! But, you will see why you need to camfer the upper and lower edges, and 45 the ends. Get that done and we'll go from there.
Amakarevic, Any type of asphalt or modified roofing systems requires there be no right angles on the roofdeck in order to avoid stressing and cracking the system. Cant strips provide a way to break a 90 into two 45 degree angles relieving this stress.
What Tinner is trying to explain to you is how to make cant strips out of osb very simply and inexpensively. Once that is done, you nail a 40# base to all of the framing and decking and install your curb flashings over that. His other drawing was explaining how to cut and wrap the material around the curb once the cant strips are installed.