I have a 5 year old German Shepard and I've put the MB mesh guard up between the rear passenger compartment and the boot which came with my GLC but it isn't as sturdy as a metal one, but its better than nothing. It can be lashed to the cargo rings on either side of the rear wheel arches. When I purchased the car I had the dealership throw in a MB rubber boot mat and and a MB bumper protector which can be folded up into the boot itself.
I did consider a crate but didnt want something that cumbersome ion the boot all the time or the faff of taking it, replacing it etc, when the dog doesn't travel with us.
MB dont do a metal dog guard for the GLC and looking online at other company's that do dog guards, they look rather cumbersome and not of good quality.