I had two 12 inch pioneers in my car for several months. I made some little cheap boxes just for somewhere to put them. I found out by experimenting around that if I put the boxes toward the back of the trunk it worked alot better soundwise. Here's a picture from several months back though.
"I'm not driving too fast...just flying too low"
-03 SB Cobra vert- 2552 of 5082 Born 02/25/03
90 7up vert - new project-07 DSG Focus -DD