I use aluminum foil cut larger than the side to be backed (use scotch tape to join additional sheets if needed) I then take several shades of spray paint and lightly spray a random pattern over the foil (not while its on the tank). Tape the foil to the top of the side and then start gathering the foil. The tighter you bunch the more granular and smoother the appearance. Larger bunching yields a 3D type of effect the appears deep with ledges, outcroppings and insets. Depending on the colors you choose it can look really natural or just plain wild. Of course you could just leave plain for a mirror type effect. I put a touch of tape arround the perimeter to tack it down. This is really inexpensive and quick, which means you can easily change it to a different texture.
