Ok, just to quickly answer the questions:
- boot is Kofferraum, there is no other term used frequently in Standard German.
Kofferraumwanne
A "Wanne" is a "bowl", not a carpet. A Kofferraumwanne is therefore a rubber/plastic mat with some 3-5 cm "walls" on the side that prevents even liquids or dirt leaving the mat to the valued carpet of you car.
Kofferraummatte
Well, that's a boot mat
Kofferraumteppich
That's the lining a car has from stock. So if you see them on ebay, they are spare parts to fix a car, not an additional carpet to put on top.