I recommend a 3 step process, 2 of which have already been mentioned.
- Aluminum rivets, no question. The dissimilar metals issue and potential corrosion of the mast are paramount considerations.
- Insulate the mounting bracket with a rubberlike gasket. UHMW has been mentioned as has polyethylene (bleach bottle). I use a pricey flexible polymer gasket material available at McMaster-Carr. I don't endorse the use of polyethylene due to UV degradation. The critical property of the gasket material is that it cannot absorb and hold water, must be impervious. I gasketed everything that had any sort of mounting base, winches, spreader heel brackets, antenna mounts.
- Apply a generous amount of anti-oxidant compound to the fasteners, rivets and machine screws alike (I'm not a fan of sheet metal screws in aluminum). The compound I use with excellent long term results is NOALOX available at Home Depot or their competitors in the electrical department. It's a Code requirement for aluminum wire electrical connections. Good stuff.