[Whiskey]

Attached is the finished lower bracket, during and after install. The second picture shows how you have to slip the back part of the bracket under the fin, and it's finished install position.
Edit: I shaved the leading edge of the rivet head to a "D" shape to give more room for the mesh. The leading edge of the bracket is about 7/16th from edge.

3rd picture. Cut a 6" x 2" piece of carboard, press it along the inner scoop., and get your 1st bend point. Then take the other half, line it with the top of the new side duct and find your other bend point. Replicate with metal. Upper bracket was formed with a vise, rubber hammer, pliers, and my hands. Make both left and right hand pair of brackets at the same time for continuity. Form the whole shape first as a 4" piece then cut in half. 2" wide piece of metal, installed with 3 #8, 3/4", self tapping screws. Cut the lower part of the bracket, as needed, so a speed nut can fit over your hole.

You have to use blue tape to hold any bracket in place prior to making a hole. First tape up the bracket(s), briefly hold up side duct to check alignment, then drill screws through the tape and metal if all points align. Loosen screws to remove underlying tape.

Note, be very careful with self tapping screws. I predrilled smaller holes then only use a drill to screw until they start to move into the thread. After that, use a hand screwdriver to finish tightening.