Use DAP Weldwood (green can) from Home Depot. It is water based and goes on with a thin nap roller (like the kind used for house trim painting). Just apply a coat onto the foam, and then one side of your balsa. Allow to dry for ~40min so they are dry to the touch. Then carefully line up the 2 pieces at one edge and apply the balsa with even pressure. This method is the best I've seen to date (you cannot remove the balsa without ripping off foam!