I have a 00 Extended cab. In the process of collecting audio upgrade for the truck. I have a 12" kicker Solobaric L5 that I was planning to use. I am no custom box builder and I can not find any thing for the square speakers for under the rear seat. Do any of you guys have a square Kicker under the rear seat?

 

This box is for two 12's and I'm not sure if it will fit your sub 100% because I'm not sure of the specs but sub boxes like this one are easy to find for our trucks. I actually just bought a new box myself a few days ago.

Ground Shaker NCHVPK 2 12 12" Dual GMC Car Subwoofer Enclosure