I had a cargo net from another car that I managed to make fit in my Volt but for the ELR, Cadillac has this thing called a cargo organizer. Go to Cadillac.com and click on th ELR and then on accessories and you'll find it. A little expensive at $130. But it is very well made and has multiple adjustable compartments. It will even lay almost flat when not in use so it doesn't take up much room. Dealer had it include in the price of the car. I would have never bought it on my own but now that I've had it a while I'm surprised how much I use it. Over all I'm glad I have it.
Yeah I know about the container solutions, I want a net like my 2014 Cruze Eco has! I like twisty roads and "sport mode" too much for a box that is going to fly around all over the place and dump everything out.