Or you could go with one designed for the grand cherokee and made by mopar. Check out the german accessories brochure
Dog guard / luggage compartment separation from high quality material,
black, accurately adapted to the Grand Cherokee, separation between load and passenger compartment. Easy to install.
Item no. (K) 82212304
You can buy them online at a few places if you put in part # 82212304, it retails for $289 best price I found was $190, but I called the dealer I used to work at and it shows up but it wont allow them to order it because it says export market or something like that.
Here is a shot of the page.