Hi I need to be able to drill a 5 inch hole in a fiberglass shower surround. The fiberglass is approximatly 5/16 inch thick. What should I use to do that without cracking or tearing the fiberglass??

When we install shower valves and heads we use hole saws. To check out a 5" hole saw click on; http://www.onlineindustrialsupply.com/hss-jewelers-saws-hss-jewelers-saws-5-inch-diameter--1-2-inch-hole.html
Good luck, Tom