The fluted rubber mat is great and gives a nice original feel, but as posted it is dear, I haven't been able to find a cheap source. As mine is a working vehicle I just have a strip of old carpet in the back, would be non slip for the dog too!
Under it is a sheet of ply about 6 mm, and under that is a sheet of soundproofing mat to keep the noise down on long tows. The plywood is good, it protects the mat, but if you don't have a mat it protects the ali floor, which is quite soft. I have seen a neat hole punched in it when someone loaded an engine block with a stud left in it!