Epoxy or silicone, I favor epoxy. You will need to make sure everything is absolutely clean and you will probably want to take some sandpaper to them to provide a better mechanical grip.
This is the best plastic epoxy I have found. JB Weld plastic weld. There are others if you can't get it.
So take it apart. Clean it especially if there is any soap. Let it dry. After it is dry it might not hurt to do a light clean with rubbing alcohol. Then epoxy it back together, per the epoxy instructions. This type of epoxy is very noxious. Do it outside and leave the top outside for a few hours. It is also best to not touch it, use a stick.
Other notable runner ups: Fixing LED light strips that separate from their adhesive, holding guitar nuts in place well enough for daily use but still mallet-able to remove it, and of course for gluing ones enemies eyelids shut