Hi and welcome,
Your combination rivet/nuts are a wonderful invention, allowing you to use machine screws in material that would otherwise be too thin to support threads. They are also handy for stripped-out holes -- you overdrill, install the rivnut, and put in the original-size machine screw.
Unfortunately they are a bit hard to find, and stainless ones are even rarer than aluminum ones. The aluminum ones are quite fabulously strong, but for a vang load, which is high and in tension, stainless ones might be better. Lots of results come up when you search; here's one:
http://www.rivetsonline.com/rivet-nu...ivet-nuts.html. Note the grip length, which needs to be greater than mast wall thickness.
Fari leads,
Brion