Thanks to latoracing for providing me with some pointers on how to form the flanges.
Below are some photos of the driver's vent that I've been working on. I need about 3/4 of the circumference on this vent and a little less on the passenger side. I decided to go a slightly different route and form the needed flange out of two pieces to see if I could avoid fabricating forms. I used primarily a bench vice, two pliers (one positioned on each side of the bend radius), a hammer, and some scrap pieces of metal to help form a sharper bend radius. Oh, and some cardboard to make a template (only a DiGiorno pepperoni and cheese box will work here).
The final photo is of the two fabricated flanges held in place only with some welding magnets, so they're not completely flat.