Once I measured and determined where to cut the hatch, I drilled a few tiny holes where I knew I wanted the plunge cuts. Then I set the frame to the side and flipped over the ply. In retrospect, I don't know why I flipped over the plywood. I could have done it from the bottom side.
I used the circular saw to make the plunge cuts. Again I was worried about tear or breaking the edge of the cut as I finished the cut. My solution was to use a crosswise piece to hold the cut out hatch.
That worked really well.