On my 13 inch dob (useing 16 inch sonotube), I cut 5 baffles out of hardwood doorskin on a scroll saw and hot glued them into place. I then cut black felt material a little longer than needed, and used a spray can of 3M general trim adhesive to glue the felt in place. While the glue is still a little tacky take a sharp razor knife and cut through both layers of felt where they overlap for a perfect seam. The baffles not only help block light, but make the sonotube strong enough for me to sit on, and ensure that the sonotube is round.
Ron P.