With the flanges clecoed to the underside of the roof, I used a knife to score the aluminum along the straight edges of the flange. Then I snipped away at the round parts with a nibbler until I could get the long edges 'isolated' so I could then fold them over and snap them off. Gives really nice straight edges, but in image 2, there's still some filing to do.
Using a knife to score the straight parts
After removing the round edges, and snapping off the scored material.
