Impaired bunnies cannot always maneuver a turn-around inside a small box. Even a one-inch threshold is too high for some rabbits. In these cases, a flat area is needed that has no barriers to cross, that can be urinated on, that absorbs moisture, and that won't scatter. And you need protection for your floor. Use a large piece of synthetic fleece as the top layer.
Synthetic fleece provides cushioning for bunnies whose bodies keep contact with the surface.
Another option is to use disposable sea-grass mats. These are fairly inexpensive and can be found in import stores such as Pier One. They are sold as flooring and can be cut to fit your needs. The middle layer can be made up of newspapers, magazines, junk mail, paper towels, and pages from old phone directories. These are available in auto supply and hardware stores everywhere and come in a variety of sizes.