kp91· Marine Engineer
If you're using 2 pieces of plywood, I think you'll probably be in pretty good shape. The alternating direction of the layers in the plywood will tend to make the top more stable, and help keep you flat. You can seal the bottom face with poly, and that should slow the rate of moisture absorbtion the way laminating would. The biggest concern to have is making sure your top stays flat when you glue the two sheets together.