I've become quite adept at building boxes. I'm on my second for my car, and have done a few others
Use 3/4" MDF.
The base should be no wider than 33" or shorter than 32". 16" tall is good. The wider you make it, the more presice it will fit, but it will also be harder to get in position. Don't worry about bolting it down. I've never had to and they've never slid on me.
Use GE Clear Silicon Sealant/Adhesive II.
The slope of the backseat is 27 degress.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>