I did a similar thing. I got the bracket for the outside grille you can buy, got some round bar and bent it into a 90 degree angle to hold the shelf. I just got a shelf from Home Depot, mounted some pipe brackets on the bottom of the shelf so I can slide the shelf on when I put up the bracket. The bracket folds flat to store it. If I get a chance I'll take a pic. It's not as nice as yours, but it's functional.
Model X19H Expandable
GMC Sierra Pick up
Been a camper since I was a kid, and that was a long time ago :)
From pup tents with no floors, to tents with floors and flies, to a pop up, to the expandable.