Bigger speakers/Improving sound in rear of Octy Hatch?

Hi all,

Ive changed the speakers and head unit now in my 2002 Octy 4x4 hatch but Im not happy with the rear speaker setup. Im sure the rear speakers are simply too small (5.25" I think) to create a decent sound field in the back of the car. Its primarily the rear most position either side of the boot cover, mounted up under the plastic panels either side. I have disconnected the rear treble units totally as I found they were simply too much for my ears when in the driving seat.

So is there anyway you can either actually increase the size of the speakers (but keeping them in the same position) or improve the quality of the speakers. The current speakers dont have a quality issue as such but they are trying to move back and forth in the very large boot space and therefore sound tiny with very little bottom end definition whatsoever.

Has any one managed to 'box' in these panels holding the speakers? Did it help?

In case it matters; hu is an Alpine 310, front components are Alpine DDD drive and rears are Kicker KS55.2 coaxials.

Thanks in advance

Lee