[nelsondog]

I have done an in care IB set-up and I would encourage anyone who wants the best bass money can buy to do the same.
I'm using an NOS pair of Focal designed specifically for IB use and couldn't be happier.
No punching extra holes or cutting of any sheet metal, just blocked off the opening between rear seats and trunk with a 1" MDF baffle. The only holes I had to block were the the openings for the 6x9" original speakers on the rear deck.
The result... effortless bass as deep as you can have and very subtle at the same time.
Don't get to caught up in sealing small cracks and damping the trunk. It's not needed.
Just do it!
You wont be sorry!
Brian

 

[nelsondog]

Justify the cost? Just get a scrap piece of mdf cut out a baffle and judge for yourself. Might cost you $10 and your time. If you like it, it may cost you a few more bucks to make it pretty with carpet
Try it!

 

[nelsondog]

What brand are your 12s?