I have done alot of reading on this subject and asked a shit ton of questions to alot of people before we built my box. (Thanks Chris) Here is a couple of pics of it. We used a basic rectangle box with a angled front that follows the angle of the back seat. This allows me to put the seat back in and still be able to play that BOOM BOOM POW... We cut out the big section of the bottom to allow us to fiber glass the tub into the box to add additional volume to the box and still have room for all the equipment in the trunk. Both 15's and the port are forward, with port on driver side. we ended up with about 7.75 cubes after displacment and tuned to about 31-33.. We have added additional braceing so tuning has changed a bit. I would highly recommend sealing off the trunk. Best 10 dollars I have spent on the whole build. ( exspanding foam FTW)
I personnally have not had any issues, in fact right out the gate we got a 149.4 (not offical) back it up in comp with a 149.2.