My tube is tied on with sisal string, which is safe for Hamsters as long as it is untreated sisal string. My Sisal came off another hanging thing I bought that was rubbish so I binned it, kept the string and tied the cardboard tube up with it.
I've just taken a photo of the top of our Charlie's cage (because he was up there lol) so you can see what it looks like.
I found it hard to find untreated sisal string in the right width and so on, but there's quite a lot of it comes with these ripnrun burrows which aren't too expensive. They are the things I bought and binned and kept the stgin! They're a nice idea but had to many sharp bits in/on for my liking - but the string is great! And plenty of it because you get 3 of them in the pack.
Supreme Rip n Run Burrows Pack of 3
PS I've tied it in a reef knot so it's secure and doesn't come undone! Holds all Charlie's 220g nicely!