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.

Note:1/4 Imported Hex Caps will be fully threaded up to (including) 1/4-20 x 1. Anything longer will have approximately 1 in. of thread.