How has no one mentioned the Elemmat yet? If you don't want the back of your E to look like a Home depot experiment or a bathroom, get something that is meant to belong back there.
I agree, if you want something to look nice, you must nicely install whatever accessories you choose to put into it. My home which incorporates many items from Home Depot including carpeting, looks very nice. So does the back of my Element. Diecutting a piece of rubberized flooring material doesn't make it more "meant to belong there" than accurately cutting a price of another floor covering material such as carpeting. A floor is a floor. What's important is the particular application and the characteristics of the floor covering.
If I were primarily transporting heavy flat bottomed objects, the Elemmat would be near the top of my list. It does protect the interior and is less slippery than the god-awful slippery Element original floor, but it doesn't do much to stop tipping or sliding of smaller items, nor does it provide any restraint of things that can roll - like 2 liter soda bottles.
A price of carpeting, if fitted with the same care as the Elemmat looks like a piece of automotive carpeting, no better or worse, and its pile provides something for a velcro hook strip to latch onto. That enables a folding cargo bag to be used without using cargo nets, tying it to hooks or sticking something to the interior panels - a simple solution which prevents my groceries from rolling all over the open cargo area.
Given my druthers I'd have flooring similar to the Elemmat in my kitchen, which doesn't accelerate, brake and change direction (so far). I can afford Elemmats, they just don't match my vehicle accessory needs.