Got my Hexomat cargo liner today!
Here's the cargo mat situation for the A3 Sportbacks in the US: In Europe, there is apparently a structural difference in the rear cargo area floorpan between the front-wheel-drive and Quattro versions of the A3 Sportback. In the US, all of our A3's (Quattro or FWD) have the same cargo area shape and floorpan as the Quattro models do in Europe. So, when Audi originally brought in cargo area liners, they were the wrong ones and didn't fit properly. (This seems to have been corrected now - the original (wrong) part number ended with '180', the new (correct) part number should end with '181'.)
While this was being straightened out, I looked around for other options. One concern with a liner that has a lip around the edge to trap spills is that the lip obstructs the cargo net attachment hooks. I've been happy with Weathertech products in the past, but they don't make a cargo liner for the A3 yet.
What I did find was the 'Hexomat' products from AutoSportCatalog.com They have both floormats and cargo liners in black, grey and tan. I was afraid that they had made the same mistake that Audi did, but they were nice enough to send me paper templates of the mats to check sizing before I ordered. The templates were the correct shape, so I ordered a cargo liner. I did not order floormats because they are exactly the same shape as the original carpeted mats that came with the car. This may appeal to some people, but I think that the driver's mat is too short and needs to reach up beind the pedals to trap the maximum amount of dirt, and the passenger's mat really needs to cover the wheel well (the Audi 'Euro' rubber mats do exactly that, which is what I'm going to get for the floormats).
The Hexomat products have deep dimples over the whole surface of the mat instead of a lip around the edge. The advantage of this is that spills are more likely to be trapped near where they occur rather than spreading around the whole mat.
The Hexomat cargo liner arrived today. I'm happy with it. It needed to settle a bit to flatten out after being rolled up in the shipping box, but I helped it out by rolling it the opposite way for a bit. It also didn't fit exactly right in a couple of spots, but removing about a millimetre of rubber with a razor blade solved that, and I'm making the company aware of what they need to do. It's a very heavy grade material, and seems like it will be even more durable than my old Weathertech one was.
Black is definitely the right colour - I think the grey is a bit too 'milky' to match well with the rest of the cargo area.