Cardboard bottom board
It's high enough for a cat to sit upright - something which is often a bit problematic with backpacks. It's also very comfortable to carry and a cat can sleep comfortable.
However - which is why my mark id low - the bottom plate is not polypropen inside but cardboard. As soon as your pet pees or worse, poops, the plate is wasted. And worse. This is what makes the backpack's stable and it can't be used without. A disposable carry was not what I meant to buy.