The photo was posted by someone who was selling the figures, so I don't think it was super glue. Maybe hot glue would hold them without permanently attaching and damaging them? I just got a collection of about 200 of them with duplicates mixed in, so I'll have to test it out and see what works on the dupes. I love how the UDM figures look, I just wish they were easier to display

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Edit: I just found this, it says non damaging so I might try it out https://www.amazon.com/Quakehold-99111-Collectors-Museum-Putty/dp/B000FJU290?th=1

Assuming you leave the containers in the cabinet when you retrieve stuff out of them, a different approach you might consider is putting a few dots of Museum Putty under them. It’s a sticky putty that’s non-damaging, removable, and reusable. We use it under anything fragile we don’t want our cats to knock over, and it’s great stuff.