The only thing I use is an old thin duvet when I need to protect the boot, I’ve used one for years especially when carrying bikes as they tend to be a bit greasy and it’s worked perfectly
The duvet is long enough to protect the back of the front seats when the rear seats are folded flat and wide enough to protect the sides too
This method has always kept my various boots & carpets immaculate
I use the box under the boot floor for loads of things; tyre pressure gauge, cordless tyre inflator, ice scraper’s, snow brush, glass cleaner and cloths. I find it very useful and use it all the time as it’s a perfect little hideaway so any mat that makes it tricky to access wouldn’t work for me