Depends on what you want to spend, like you I wouldnt want to cut into the panels themselves for speaker, the only suggestion in that would be that, you could get some fiberglass panels made.
For the front of the car in the kick panel area, and a new panel for the rear of the car for something decent.
Kicker makes some good small 6 inch bass enclosures that you could put behind the front seat.
I prefer Kicker for all the components, they make great amps, components and subs.