Stainless rivets and seal the mast base to the mast with 3M 4200 sealant (usually available at wal-mart). If you use 5200, you probably won't be able to get the mast out again.

either the 4200 or 5200 will help work against galvanic corrosion.

Use stainless (or monel) rivets where there is a high loading. Aluminum rivets are very weak.