Although I historically use S77 for joining winghalves together, a Polyurethene glue called Elmers ProBond works great as well. A little bit on a slightly damp surface will cause the ProBond to foam like crazy and force itself into the foam for quite a ways. Also great stuff for glueing in spars with but you need to be careful about not forgetting to tape over the glued area to prevent the foam from going everywhere. It's sandable, just not much fun to do.
I went on a shopping spree when I found out they were changing the formula. Home Depot in Colorado still has the old stuff on the shelves as of last week. If you hunt around the shelves you can generally find it behind everything else.