The tube is held in by the Sputnik, the Sputnik top comes off so it's easy to secure in. You'll see what I mean when yours arrives, it's hard to describe!
Re the monkey barring, they can hurt themselves falling from the top of the cage so it's a good idea to add in something to mitigate this - I've used a grass mat here (unfortunately he chews it so it's not a permanent solution for him) but whatever you use it should break their fall so thei don't hit anything hard. I've set it up so whatever stupidity he gets up to monkey barring he either doesn't fall far or falls onto the soft substrate. This is Glowy's current set up:
I'm going to be adding in probably another Sputnik behind the one he has, and a hay house by the ramp.