The rear Polk 4 Inch DB401 Speakers didn't last long so I put in Blaupunkt 4x6 GTx-462 DE Speakers. I paid $42.99 plus shipping from Amazon. They're rated at 35 Watts RMS (160 Watts Peak) and have a better Frequency Response of 80 Hz to 22 kHz.
I decided to mount the speakers flat against the Grill. I cut the old mounts for the 4inch units and used them to secure the 4x6 units across the longest dimension of the rear opening. I then sealed the air gaps using cut pieces of Corrugated Plastic (old Garage Sale Sign) secured with Goop Adhesive. I suspect the Voice Coils may last longer due to the flat mounting versus the angled OEM mounts. The Blaupunkts sound better than the Polk Speakers probably due to the better frequency response and higher peak power handling capability.