I would simply get some thin material, as cited in the post prior to mine, cut, sand edges, paint, and then fasten in place with caulk or silicone. You can cover the entire back, but it will look fine if you simply cut and finish a piece big enough to cover the hole. To finish around pipes, cut clean holes in your piece and then cleanly cut the piece in two halves and then glue on the finished pieces around your pipes. I had to do this with our new kitchen vanity to accommodate plumbing.
Sounds like you want to put the supply hose in the wall. You may need to remove some drywall, put in your hose, and replace drywall - see http://www.drywallinfo.com for help on this. If you are lucky, perhaps you could fish a stiff wire through from hole to hole and then pull your supply line through and this would save a lot of work.