Tough problem, Gene. Including photos of the problem would be helpful. Although this does not apply directly, the spacer I made to keep from bumping my noggin on the opening knob for the new vent on my '78 Trill might help?
It was cut from 3/4" ply and painted. Butyl tape, 5/8 screws from the inside and same from outside to install. If I were installing a large vent and cutting out the raised/reinforcing area I would consider making a frame out of ply and also reinforcing it with aluminum angle on the inside bottom edges for extra stiffness...
Hope this helps. Good Luck.