Seems like lots of PO's do what they can to make the problem worse. Mine did a "repair" with fiberglass.
There are several ways to fix the mess and most involve some welding. I was able to fix mine with fill welds and small repair patches cut from 18 gauge steel. Worst case is to replace the whole flange from a donor car and a new upper trunk panel. After repairing mine I smoothed the surface with Met-All filler
The rear deck filler panel would be the way to on it ,(IMO)
The sides ,or top if you don`t have to replace the 1/4s i would cut out a section and either make the piece or go to a metal shop and have them bend a piece in the same contour that you have , then start cutting out the old and welding in the new. Just make the pieces extra long that you make or have made , then start welding them in , Good luck on what ever you do and keep us posted .