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.
1/2 Imported Hex Caps will be fully threaded up to 1/2-13 x 1-3/4. Anything longer will have approximately 1-1/2 in. of thread.