Best glue to reattach lens?

Red tape is the answer to everything:

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Though a quick glance at the teardown suggest there might not be enough room for red tape, so black tape may have to do:

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Black tape is stickier but less permanent. Red tape is similar to what most manufacturers use when assembling phones. Black tape is similar to what they use to waterproof phones. It may be worth it to try both and report back so we know for future reference.