Stayhold Trunk Organizer
The Stayhold is an ingenious divider that can keep things in your car trunk from falling down or rolling all over the place. It works great for groceries, toolsets, paint cans, and a lot more. You can reposition it as needed, but it grips the carpet wherever you place it so it won't slide around as you drive.
You can get a single gray Stayhold for $13, but I recommend getting the $25 twin-pack, which is available in: