Blacktak

Blacktak™ Light Mask Foil is a trademark of City Theatrical. Black aluminum foil tape, 2" wide, with an adhesive on one side, it can be used to mask undesirable light leaks in the presence of high temperatures.
5048.jpg
3600_blacktak_2_light_mask_foil_med.jpg
If budget and/or availability is a concern, look for "aluminum muffler tape" in an auto supply store, although you'll forfeit the black, as muffler tape is reflective and silver on both sides.

Often used in conjunction with BlackWrap.