http://www.delica.ca/forum/l400-bed-shelves-14517.html
My version isn't quite as elegant but it does the trick for me! I'm planning on building the slide out shelf eventually as well, would be very handy to get at things that have slid up behind the seats.
I have a permanent access to unistrut so that's what I built the side brackets from, plus it's full of holes if I need any bungee holds!

IMG_4530 by TheMacrobius, on Flickr

IMG_4531 by TheMacrobius, on Flickr
This was a bit of a pain.... I wanted it a tight fit so I cut it EXACT. Big mistake as I kind of forgot the interior is contoured. Ended up being about 56.5" wide and 42" long by the time I was done trimming. I made templates for the hatch side out of cardboard so I could cut out exactly what I needed.

IMG_4544 by TheMacrobius, on Flickr
Got to use my new Gearwrench socket set too!!!




IMG_4551 by TheMacrobius, on Flickr
Bought some indoor/outdoor carpet from Home Depot and wrapped the plywood. Spray adhesive first, then wrapped around to the back and stapled. Damn that wrinkle!!!

IMG_4554 by TheMacrobius, on Flickr
Used 3/8"x3" carriage bolts with fender washers and wingnuts on the bottom. Only 4 bolts if I ever need to take it out.

IMG_4557 by TheMacrobius, on Flickr

IMG_4558 by TheMacrobius, on Flickr
Finished!!

IMG_4563 by TheMacrobius, on Flickr