I'd use machine screws, tapped with a good quality tap (not hardware store: go to the places toolmakers buy taps)
I'd personally go for decent quality 316 (with nice shiny rolled threads) because in small screws like this, stress corrosion cracking is quite an issue from the roots of the threads, and 316 is less prone.
And I further agree with Nick on Tefgel: plus it doesn't hurt to first run a couple of turns* of PTFE thread tape onto each screw, including the underside of the head
(*no more or it'll push all the Tefgel through, and strip itself off)
The biggest advantage over Monel rivets (which would be my second choice):
Machine screws can be removed (and should be#) at every refit
to check for weakening - pick a couple of the worst looking ones and bend until break. (Keep a few new screws of the same batch as control samples)
# This is the only way to be sure they do not seize into the holes eventually and become a liability: it's rather difficult drilling out stainless fasteners in a softer substrate, even if you take the mast down first. It might be worth springing for recessed hex fasteners: they're MUCH easier to drill out, as the drill self-centres)