Why not use aluminum rivets? - they're not carrying any load there and the like metals will be much less likely to corrode.
Good point, as I recall these rivets are more for holding the end cap in place and would note take too much load. When reassembling my boom I had to rubber mallet the end caps in.
My suggestion for Monel came from something I had picked this up from a AP & I when I use to own a Piper. Almost as strong a stainless and won't cause corrosion like aluminum.
