I recently got an avant and went through the same thought process for my Labrador. In fact I asked the same question on here.
I considered some of the boot liners that only cover the floor, but on my wife's hatchback the sides of the boot became matted with hair so I wanted something to cover the sides of the boot as well.I just shelled out £185 for a boot divider so didn't want to spend too much on a boot liner.
Audi Dog Guard, Audi A4 Dog Guard, Audi A4 Avant Dog Guard
You'll find most of the made to measure boot liners which cover the boot floor and sides cost in the region of £70 to £90.
I've decided to just buy this generic universal one off amazon and I'm hoping that does the trick. I'm sure it won't compare to the more expensive ones but for ten quid you can't go wrong.
http://www.amazon.co.uk/Sakura-SS4612-Universal-Boot-Liner/dp/B006FXLP28