I am building a custom box for 2 15 inch Kicker

Hello bgl6176,

According to their specs, the L7's will work in a ported enclosure with as little as 3 CuFt and up to 6 CuFt. Usually with ported, bigger is better, so let's use the 5 CuFt per chamber. I'm assuming that you have room for a 10 CuFt box in the trunk? That's a big box? Almost half as big as a medium sized refrigerator. I hope you don't plan on putting your golf bags in there too. For 10.15 CuFt, a box will need to be 36X29X20, allowing 0.15 for the partition, sub, and port displacement.

To tune each chamber to 30Hz, you need a 4" diameter port 4.6" in length. Lots of custom builders use PVC and even ABS pipe for round ports. You can also buy "pretuned" ports from sites like www.partsexpress.com.

You'll have to measure the exact room that you have available and then adjust the dimensions accordingly.

Here's a link to a great site for calculating almost anything in box building and subwoofer related. You can use the old "multiply LxWxD and divide by 1728" and the other formulas too, but here you can plug the figures in and immediately see the results.

http://www.the12volt.com/caraudio/boxcalcs.asp#rec

Hope this helps.