We usually use superglue, but there have been people in the past that were epoxy crazy and used it here. In 1979 it was probably superglue. If it gets a good whack, the brittle bond can break. but that's good... just pull it out, add a little more glue under and replace.
You could use a double sided tape like carpet tape if you like.
You can also shim stuff along the edge to wedge it in place - veneers of wood, matchbooks, whatever works.
This is for hum cancellers between the pickups, not like the ones mounted in the cavity like Keavin's above.