It might be this mat but I purchased a approx $200 mat that folds out to cover the back of the seats when folded down. I have probably used it about once every 2 months. I used it for carrying stuff to the dump but now use other vehicles for that.
A dropcloth, for this occasional use, would work just as well.

I use the WeatherTech one, fits like a glove but covers the 4 corners tie-down thingies.

There are cut-outs for the tie-downs at the corners at the WeatherTech mat, but using them compromises the ability of the mat to trap liquids.

Are you referring to the mat that is folded onto itself where one side is carpeted and the other side has a vinyl like waterproof surface? I believe this is called the reversible luggage compartment mat. If that's the one you're referring to, I have it in my E70 and can take a couple pictures. The GM at the BMW dealer where I bought the car liked it so much, he got one for his car too.

I just measured out the boot area and went to the local hardware store and cut the rubber lining to size as they have it on a massive role.

Cost me a total of $7.

I also use the WeatherTech cargo mat, my first experience was my wife's 2002 CRV (I got the mat from a guy who sold his car). I even had a small gasoline spill on the mat (from a car refill can) once. It held the gas, protected the car. It didn't melt or alter the mat at all and it cleaned up fine - no latent small after a day.

I was convinced. I've put the same in my E53 X5.

I also use the WeatherTech cargo mat, my first experience was my wife's 2002 CRV (I got the mat from a guy who sold his car). I even had a small gasoline spill on the mat (from a car refill can) once. It held the gas, protected the car. It didn't melt or alter the mat at all and it cleaned up fine - no latent small after a day.

I was convinced. I've put the same in my E53 X5.

I went with the Weatertech cargo liner. Like except for when I want to use the third row seating. Then everything has to come out of the back including the liner. If on the road, what to do with the liner other than rolling it up. Big pain.

You use the third row seating? That's a much larger pain in the b*tt than the cargo liner... :)

I don't have loose items in my trunk area. Using a collapsible cargo bin makes it really easily to get all the stuff out and back in in one go. I have something like this http://img.auctiva.com/imgdata/1/5/4...9194365_tp.jpg

I have, 51 47 0 444 754, Fitted Luggage Compartment Mat.
$100 from BMW of Towson via Amazon. Amazon.com: Genuine OEM BMW Fitted Luggage Compartment Mat - X5 SAV 2007-2013/ X5 M SAV 2010-2013: Automotive
The mat really works well, I had probably 16 ounces of liquid spill in it without making a mess in the car.

http://ecx.images-amazon.com/images/...3L._SX385_.jpg