I didnt make plans per say, the box is 6" deep, like 22" long and 14" wide. for carpet go to Parts express.com. I got 6 yards for like 24 bucks. also theres a divider wall in the middle. I used 3/4" MDF to build it with, and used 2x2's in the corners for support.
The box is very strong, I've had two people sitting on it. I used some plumbers wrap to attach it to the stock seat mounts. it works great, you just have to be creative with the seat back.
My 92 YJ
Clarion BD216, RF 200aIV, Clarion APA2030, RF HE2 12's, Rf 4x6's, RF 1" Tweets, Eclipse EUU-1300, Aura Bass Shakers Security: Clifford Power: Optima Yellow top,Thrush turbo 2 1/2" Exhaust, Radio Shack Sport CB, 102" whip antenna, Dual tips on Exhaust, K&N custom intake, KC Lightbar, 4 KC Daylighters.
Scored a 139 on the Jeep Rater.