I managed a 6" Boss 2 ways in the stock location-- you'll have to move
the center of the 6" speaker forward slightly from the  center point of
the 4: stock speaker,.

Make a sheet metal flange large enough to be attached to the door panel
mounting holes in the speaker area.  This allows the 6" speaker to be
recessed into the door.  Then mount the speaker to this flange, by
cutting the standard opening hole in it and attached the speaker and
grill.

Then cut the opening in the door panel large enough to go over the
entire grill. Don't cut the fabric all the way back to the opening
circumference, leave an inch or so. Then you can glue the fabric to the
back side of the panel . Then the edge between the speaker grill and the
door panel will look nice and tidy.

Sincerely,

Mark Keller
91 Carat
Cowichan Bay, BC