I have one of these and love it:
It has ruber feet to keep from sliding around when loaded and folds up nice and compact when I'm not using it. It's big enough to hold a few grocery bags or at one point I used half of it for a washer fluid bottle, jumper cables, shop towels and other stuff. It's really handy and it comes out of the car and doubles as a way to carry stuff. Any Subaru parts department should have one, or get one for you.