I received the 4x6 speaker from Aicraft Spruce
http://www.aircraftspruce.com/catalog/avpages/quam_speakers.php.I was able to use the steel frame from the old 6x8 speaker as an adapter.
I cut out the old magnet with a hacksaw, drilled some mounting holes, bolted the new speaker onto the old frame, soldered the wiring onto the speaker, and mounted the new speaker in the plane. It looks good, and it works.
When I install the headliner, I'll need to carefully cut some small holes in the headliner for the speaker.
