I got this trunk tray (the Large size; 28-1/4" wide) from the Container Store and I love it. It's plastic and rigid, so it holds its shape (vs. collapsible/canvas solutions), and has modular inserts that can be removed or placed alongside the main tray. It fits perfectly against the rear seats in the trunk, holds grocery bags & other items, and will contain most liquids. It has "feet" that grip (pretty aggressively) the trunk floor and prevent it sliding around. Best of all, it's about $13!
http://www.containerstore.com/shop/s...uctId=10024142
__________________
2011 135i 6MT Space Gray/Black/Aluminum | M-Sport | Premium | CA | Nav
BMW Performance [Suspension, Exhaust, Grilles, Mirrors, Spoiler, Shifter/Boot]
Ordered: 11 Sep | Production: 16-23 Sep | Released to carrier: 26 Sep | Awaiting ship: 1 Oct | In transit: 14 Oct | Prep. Center: 9 Nov | Delivered: 10 Dec