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The first picture shows the template I used to mark the center of the holes. This is straight from the drawing I made earlier.  I then fitted the piece under the main panel where It attaches to the channel that runs underneath. The .063 material with the 90° bend is much stiffer than the channel it mounts to.  I think the transponder mounting will help stiffen up the channel and I may be able to add an additional stiffening piece that runs back to the second channel I installed. It will only be a problem inserting and removing headphone cords and it may not be much of a problem anyway.