On the metal and potential for it to short out, that is the issue created by tight gap or close tolerance required to work. What is directly below the hot wire MUST be a good ground contact. Whatever you put down, just make sure it will conduct electricity, since if it doesn't, it will work like an insulator and the animal won't get shocked as he passes. Wood, plywood, rubber, plastic, plastic mulch, plastic weed barriers, etc all are poor conductors so won't make a good ground. Metal is a good one. I also think 8' x 16" paving stones would work well and you can weed whack along the edge of them all day long.