I use a mat that I got at the local hardware store. It's plastic, with a raised rim and a 'diamond plate' texture. About 1.5' x3,' works perfect, and fits in the rear XL storage area. I have no idea what it's intended use was, but it's really just like a knock-off husky liner. The best part it was only $10.
Before I found it I was using a rubbermaid tub; drilled a hole in the side about 1" off the ground so it would drain.
Would send a pic, but the SMB is at Ford today