OK - I like what you're doing - It's not a hare-brained scheme that's going to look like some black-ops job.
My apologies - but some of the crazies I've seen 'on tour' in the bad lands get into big trouble.
I understand now (I think it might have been the combination of 'security' and 'travelling across Europe' that set off alarm bells.
What you really need is a rear roll cage - but they make getting in and out a hassle as well as making a large hole in your wallet.
For security the panel from a commercial is a good idea - as would be a dog guard, but wont help much if you roly-poly it into the scenery.
We like to keep it simple - a black bed-sheet stretched over the entire back area will keep prying eyes away from most stuff - the determined thief will just break a window anyway.
We do not fit any shelves or extra solid cupboards etc. The best stacking storage boxes available are the empty fruit boxes you get for nothing at Lidl. They are a perfect fit in the back of a Freelander - get some and have a game of Tetris with them. Interchangeable and adaptable - they can be added to or dumped as you go - and they dont rattle or damage the interior trim. Hold them down with bungees.
We go to the Balkans 3 or 4 times a year for a month at a time and I know how much 'stuff' needs to be packed in.
Our hobby (on the way home) is Alpine Pass Storming - usually in Austria.
Have a good and safe trip.
Singvogel.