I currently use nothing on the bottom of my QC, but I have a litter pan for them to pee in. Only 1 gets the concept, the other pushes his but up next to it, and pees next to it.... oh well. I have tried fleece in the past and it was just too much of a pain to clean. I have also tried carefresh, but they ate it like food, so then I switched to all aspen and it was just too expensive to buy a huge bag and use most of it in 1 cleaning.
So now nothing is on the bottom, which makes it easy to spot clean and vacuum, and I clean out the litter pan every 3 days. It works well for me.