the location to be joined to just switch to stainless hardware?
Is the female fitting nested into a hole that keeps it from rotating while you tighten the male end, or is there room for a multi-tooth lockwasher over a flat washer?
Just curious, are the plastic portions of the fasteners made of nylon, or of something else? I think nylon is the most breakage resistant plastic used for screws and nuts, but while it can be impressive, it isn't as strong as stainless.
When I'm up against something like this, I take the parts to a good hardware store and scrounge in their screw drawers for a possible solution. I may have to send the sales personnel away because they imagine they understand things that they don't.
Heel and Pegpads™
URCHIN Portable Anchor