Brad-
That is a nice setup that guy's got going on!
I removed the 40 at the moment mostly in prep for the VOT in Aug; with just me and the wife doing that I wanted it to be setup so a passenger could access the fridge on the move. Also illustrative that the Disco is a perfect overlanding vehicle...for 2. When I do it with 4 (no dogs...but the other three are all women...) I start facing similar challenges to you.
My first couple of Discos had jump seats on both sides (this one has rear A/C, which I'm retaining b/c it VA heat it does a great job of cooling off the truck faster...tradeoffs). On the old trucks I removed the plastic trim entirely and made a much tighter panel out of diamond plate (painted tan) with accompanying tie downs on the sills of the side windows. I found that a 5 gal scepter can fit perfectly in the space on one side, and the power tank + did well on the other. As you can see on my current application I retained the CO2 and H2O is now mostly out of either the cooler/fridge, bladders on the roof (for washing up) and a couple of 1 gal rotopax that are in the very back of the shelf near the seats in that unused awkward space.
I am thinking of going with a tilting fridge setup if I go to an application where it stays in the truck but in the cargo area.
The dog guard comes in handy for just being able to toss soft goods in on top 'less secured', adding in grid to the side windows would give even more storage-I've found that MOLLE pouches woven through the grid work well but I'm also addicted to coyote brown and have plenty laying around as a side effect from work.
Doing the rear door in diamond plate is an easy job and the add on of a table gives some good flexibility.
Adding in an 'attic' of sorts is something I've contemplated but the rear a/c duct would make it difficult to access. I think it would have more benefit on a non-ac Disco.
Post up pics as you go, always interesting to see how people tailor their Discos to their application and usage.
r-
Ray