I have one of these under the car seat in my X. The bottom is pretty rigid and does a great job protecting the seat from scuffs and indentations.

http://www.amazon.com/BRICA-Seat-Gua.../dp/B00B7M7BKW

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