Even though plywood is just veneers glued together, they are glued with a hard, noncompliant glue. Adding another layer of veneer with a different type of glue...ie., contact cement...a very flexible, rubbery glue, seems to me that there is potential for problems. This is all wonderful theory and may not be a problem at all. As has been said, it should be strong enough. I think that statement might be true. One other concern for me is that I know when substances laminated with contact cement are left in the sun they will delaminate...curl at the edges. In a kayak, covered with epoxy/fiberglass, this may not be a problem. As you can see, I guess it's just something I'm going to have to try to be sure about. My wife says I flog the horse long after it is dead. She may be right.