Match drill three equally space holes around extended flange, cleco as drilled.
Slide intake from center. This actually worked really well, as the vinyl was not attached to the intake. The intake slid right out of the center plenum, then the vinyl was removed from the inside of the flange.
Shape and smooth surface and edges of flange extension.
Install right intake and center plenum.
Position the left intake as detailed previously for the right intake.