Blacktak™ 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.
A very similar offering from GAM Products, is BlackWrap™ Tape:
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.
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