By the way, I got the cargo tech things from weather tech also (you can see just a little bit of them on the top of the pic). It's great when you have groceries or loose stuff in the car you don't want banging around while you are ripping corners
The four corners can be placed wherever you want, making it adjustable to lots of different configurations depending on what you want to secure. I've never used them before, but I found that with the M3, you can't really have stuff rolling around in the trunk!
https://www.weathertech.com/cargotech/