Take a look at the Whitecap 316 S.S. Rail Mount Rod Holders ($47) at Boater's World. They are completely stainless and through bolted so you don’t have to worry about stripped threads and such. I put them through 4 years of hard work on the old boat dragging wire lines with 32 oz. mojos and 20” umbrellas attached and never had as much as a loose bolt. If wouldn’t hesitate to use them anywhere.