Sometimes you just need to step up the program and use a big old Hex Cap. These babies have got the heft that you are looking for in marine grade 18-8 Stainless Steel so it will be able to resist corrosion. Remember that Stainless Steel will sometimes Gall (seize) when tightened under load -- especially with larger size fasteners like Hex Caps. To prevent this apply a dab of Anti-seize compound before tightening the nut.