I've been thinking about getting something like this: WeatherTech® CargoTech® Cargo Containment System | WeatherTech.com

And maybe a couple plastic bins to store my misc crap I keep in the back (bungees, jumper cables, a blanket, bag of extra clothes).

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2006 X5 4.4i
Sport package, summer package, cold weather package, DSP + Nav
Bought 2/14 with 73k miles