Thanks guys!
He's in an Elmo XXL at the moment, which I think is the shorter version of the Borneo? Glad I went for that one, since he's absolutely useless at climbing.
Someone has been tunneling under his wheel today so I need to work out how to attach it to the roof before he crushes himself
I think cable ties should be okay since he won't be able to reach them? Don't think I can screw it on without taking the cage apart...