Stainless steel is the least reactive of the dissimilar metals but generally cost more than the all aluminum rivets.
2 layers of 3/4" exterior plywood glued with Titebond III or Gorilla glue or Titebonds version of Gorilla glue. I've used both and find TB3 easier to work with.
The transom I treated with the Old Times formula was 1-1/2" thick 60" long and 16" tall. I mixed up a quart of the old timers and had 6oz left over. You keep applying it until it pools up on the wood then wipe off the excess put plenty on the edges. Then let it dry for 72 hours then apply 2 coats of straight Spar. I painted mine to match my boat after the spar.
The Old Timers formula is.
One part Spar varnish or Spar urethane, One part Boiled Linseed oil and two parts Mineral spirts.