Run a small amount of glue along the beveled edge of the speaker and quickly attach it inside the box. Hold it to give the glue a chance to dry. Secure it by adding some spots of glue along the top and bottom edges of the speaker where it meets the box. Try not to glob it on so that you don't add more weight. I ended up using way more than I should have, but it didn't make the box unstable.
Finally, glue back together the top and bottom of the box as it was originally sealed, connect an iPod and kick out the jams.