Just built this enclosure today..I have 2 more bags of Coco Fiber in the mail at the moment so I can make it an even 4" of substrate throughout the pen. It is uneven, one side has about 2" and the other has 4". Check it out!
http://www.youtube.com/watch?v=j4E0gB-DKtY&feature=plcp
http://www.youtube.com/watch?v=j4E0gB-DKtY&feature=plcp