made a larger opening with smaller radiuses, cut a new door.
Cobbled up a hinge design using cereal box CAD and welded it together.
The blue line is the factory speaker grill opening
it actual opens and closes and the radio fits
Got brave and cut up the face plate. Little trimming of the upper edge of the hole and stuck the cut off piece on with tape
Have some more tidying of edges to do.
My plan is to cut a section of the hinged 'test piece' and spot weld it to the back of the face plate . Will take a bit more fiddling to get the door flush, but this is working out so far.