sub placement in 68-72 GM A-Body Convertible

[Zanie]

OK, I'm not looking to shake all the screws loose in my 71 chevelle convertible but I like decent base.
I currently have a four speaker system: I have 2 six & half inch
Infinity speakers in the kick panels and 2 six and half inch Boston Acoustics (because of their compact magnets) in the cylinder covers in the rear.



The system is clear, but is lacking base. and doesn't look obnoxious (all the grilles, surrounds, etc are satin black and blend in fairly well).
The holes for these speakers were cut before I purchased the car.

I don't want to give up any trunk space. Mounting 6x9s in the stock position really wouldn't give me the sound I want.
I thought I read about someone making use of the area between the well and the back seat for a sub enclosure.
Just a couple 8 inch subs would surely give me the sound I''m looking for.
Has anyone done this to their 68-72 convertible chevelle, 442/cutlass, GTO/lemans, Skylark?
Thanks!