Have you tried wetting the suckers before attatching the bottle holder to the cage?
We've got two and find they work brilliantly.
Alternatively, get a small bolt, a washer (or two) and a nut and put one washer on the bolt, then the bolt through the hole. Then put the second washer on the outside (if required) and add the nut to hold it all in place.
Just be aware that the mini duna is only suitable for dwarf hams I'm afraid. Not an issue unless you want a syrian though
And I was beaten to the bolt and washers idea lol, no worries