Thanks Again to Porterfield for having the right Naca duct, and for being so close to my house. Since the piece is extruded plastic, the material is extremely pliable. Add a little bit of heat and work quickly. Plastic will contort as it cools. Try to hold the specific shape and cut the flange shape once its bent.
Skipped a few hours, but this is after bending all the tabs, cutting them, making the sheet metal mount out of alum, and drilling/riveting a pattern on the duct.
Hollowed out the 3" hole with a 1" hole saw and 3/8" bit on the left over. It needed to be filed and sanded.