If the Corian doesn't work for you, I have found all of the PVC materials (Sintra, Komatex, Komacell) to be best outdoors in the 6mm thickness. The 3mm or less and 10mm or more tend to warp easily from the heat, and all of them fade in the sun. The best outdoor longevity is black for some reason, white yellows well before it deteriorates. For this application I'd suggest white 4mm Alumalite or the equivalent, which is PVC with aluminum on both sides. Then cut the letters and borders from black 1/8" PVC and stick on. The Corian would be best, though, if you can get it.