If you're concerned about ensuring the fuselage remains straight, use a slow curing glue (titebond or elmers carpenters glue), use lots of pins and clamps to hold things in position while the glue cures, and consider adding some triangle stock along the joints to add support, particularly where plywood joins plywood at a 90 degree angle (fuselage sides, formers). The weight added will be minimal, but the strength gains will be substantial, and it will be more resistant to future warping.

Brad