Gorilla glue (poly glue) works awesome. I roll the glue on the cores with a small foam roller until all of the color is gone. Just a shine or sheen is all that's needed. Mist your skins, assemble and clamp firmly between pieces of MDF. The glue will expand into the foam, attach itself to the balsa and never let go. Even better, virtually no weight gain. This is an extremely strong bond.