There is an official rear boot liner for the F31/F30; there are a couple of different versions for SE, Modern and Sport (ones I've seen so far). They are all black but have different colour strip, for example Sport has a red strip, Modern is Sliver. They cost about £84 in the UK.
Like the OP, I've opted for the standard 18" mainly because I live in central London where the roads are terrible and with my E91 I've had to replace 5 tyres all due to potholes etc etc, none of my tyres have ever been replaced due to wear! I did opt for the Adaptive M Suspension but thats not going to save my tyres from potholes.