I use newspaper under whatever litter I am using (paper-based, hemp, etc) and hay on top. I don't like disposable tray liners. Apart from the plastic element, the ones I tried (freebies) never really worked - they don't fit the trays and stay in place. Newspaper works really well. You can just roll it up with the contents, and it's compostable. It's also less of an issue for diggers and chewers.