Here are some pics of the rear drawer ext that I built for a trip back in 2010. Made out of 12mm ply, glued and screwed together. It is held down by 2 of the seat mount bolts and 6 x 6mm stainless steel turnbuckles. It is carpeted on the outside and most of the inside. I made a door for each side with barrel bolts to them keep closed and 2 hatches in the top with flush mount latches.
I rolled out a self inflating mattress on top of the box and slept on this for 4 weeks on the trip (Oh I forgot to mention in my intro that I am in a wheelchair (no legs, well not real ones anyway) so I only just fitted across the patrol.
Since that trip I have chopped it in half so we could use the 2nd row passenger seat but I don't have any pics of this as yet.
cheers
Greg