Here is your pro tip for this one.. I bought a couple grocery bag hooks from a VW Passat and have been moving them to each new 3-series I have gotten. They are spring loaded and fold up automatically when not in use and work perfectly to secure a bunch of bags. I have 2 of them installed in my trunk.
https://www.amazon.com/Cargo-Trunk-P...R3B2HJYBP05EZZ
I use a small self tapping sheet metal screw to secure it to the top of the trunk. If you do this, please make sure that there is nothing on the other side of the body panel you drill into, look for wires, and make sure you don't drill through to the outside, be very careful. Also be sure to leave clearance for the trunk floor to swing up to the holder clip thing.
Alex.