While the idea of a steel separator seems like a good idea, I don't feel that it is your best option.
Let’s take a standard 12 foot tow chain rated for about 6 tons of pull. That weighs in at about 12 to 13 pounds. You slam on the brakes and get into a head on collision at 30 miles per hour. By the time that chain has traveled 2 feet it has become a very dangerous instrument.
Those of us on this site who have responded to car accidents have see items in the back of an SUV, car or truck become dangerous. Some time ago I stopped for an accident; it was a head on collision. One of the cars was a Chevy Cavalier (stating this only for reference and NOT for bashing or endorsing any brand of car) The person in the car had kept a spare tire in the trunk, the back seat was one of those fold down type of seats. When I showed up the spare tire that was "safely" in the trunk of he car was in the passenger side front seat, sort of. It broke the passenger seat and folded it forward.
Anything at all that has significant mass is going to demonstrate Newton’s laws in a rather horrific way when an accident or a roll over happens.
****Disclaimer: I am NOT in any way shape or form associated with any products I am about to mention, I do not receive any compensation for there sales nor am I in any contract with them in any way shape or form. I just love to look for cool stuff on the web for my cars, and trucks******
First you could simply get a truck box that is made of aluminum and put it in your SUV and bolt it down with grade 8 bolts nuts and washers. With some looking and taking precautions I am sure you can figure a place that will allow you to bolt the box down to the stouter part of your SUV.
Second: You can contact Truck Vault at truckvault.com and they can make you a custom box that will bolt into your SUV. (Again I am in no way shape or form associated with this company) I will warn you they are expensive.
Third: look into hiring a carpenter to make a custom drawer set for your Bug out Vehicle. Have a stouter drawer set put into place and again bolt it down with Grade 8 bolts washers and nuts.
Fourth: look into some jeep parts stores and buy the jeep drawers that bolt into the back of the jeep.
http://www.tuffyproducts.com/p-116-046-rear-cargo-security-lockbox.aspx (again I have no affiliation; I am just finding the stuff on the web)
If you do go with a metal screen you will need to be sure it is bolted into place so that it will not go forward and add to the chaos if you do crash or roll.
For those of us with a truck,, if it is loose in the bed of the truck and we have a sudden stop due to things like front end collisions,, well you know the math.
Tyber