How about adding Flexible Baseboard Molding Trim all the way down. That way the black mud flap will visually continue that line all the way to the back wheel arch...
My hope for a mat like this is to have a built-in flap that bridges between the edge of the bed and the tail gate to keep out debris out of that area. Holding it in place with magnets would be a excellent addition.
Good luck everyone.
Cheers