I'm going to suggest this stuff from scrapyards, it is designed to be tensioned and live outdoors.
http://www.nationwide-trailer-parts...29QD_Qp9vRorqddiDJnRrSsJiVEkx-0hs2xoCmxDw_wcB
http://www.nationwide-trailer-parts...29QD_Qp9vRorqddiDJnRrSsJiVEkx-0hs2xoCmxDw_wcB