Best glue for plywood in damp environment?

Hi:
    I need to build a base plate (25x25") out of regular exterior
plywood that will be a total of 3" in thickness.
     It will be in a damp (not wet) and dark environment, under the
center  of a gazebo, about 1 foot off the ground.
     I simply cannot afford marine plywood.  I plan to glue either 6
layers of 1/2" or maybe 4 layers of 3/4"  exterior.
     It needs to be relatively permanent.  Once in place it will be
almost impossible to get at.  It will sit on concrete (30" diameter)
and will bear much of the weight of the gazebo.

      What would be the best glue or sealant I  should use?
 
      Any tips?  I've never done this before.

    Thanks very much.

      John

        (at home now, but soon to be  at the cottage)