definitely go back to back. as other people said.. they wont ship you an 18' piece of plastic
if you get a 48" x 96" (4' x 8') sheet off us plastics or where ever, you can cut it into 3 16" wide strips adn have a 12 ft box, or 4 12" strips and have it 16 ft long.
id definitely say go no thinner than 1/4", maybe go 3/8". i personally think that 1/2" would be wayyy overkill (especially if its just you/a few friends hitting it), and its expensive.