Just to be clear what I mean... Yes you need to cut bars for these but most cage bars are easily cut and as long as you're careful where to cut you should be ok - we find using them pretty easy.
Ferplast ones
Ferplast Fpi Hamster Cage Expansion End Cap: Amazon.co.uk: Pet Supplies
I think these're the right ones from savic too (and savic and ferplast tubes are completely interchangeable)
Savic Square Connection Ring Spelos/ Metro: Amazon.co.uk: Pet Supplies
The advantages to those are that you can block them off to use the cages separately with ease and the tubes are held securely in place because once those're on cages they're exactly the same as cages like the hamster heaven that arrive with places to attatch tubes. (Plus I find attatching those way easier than I've ever found making holes in plastic.) When using them make sure you remember to leave the bars to clip them to, you need to mark where to cut on both sides of every bar between those two though and then just cut them, nice and simple
For the record we use the wire cutting bit on our pliers to cut bars - the pliers themselves were fairly cheap when we got them and they definetly do the job with all standard hamster cages. Its just a case of mark where on the bar you want to cut, put the wire cutter bit of the pliers on that mark, close the pliers (job done!) then repeat as many times as needed to remove the number of bars you need removed. Its much easier than most seem to think cutting bars would be
(And we've done this with a lot of cages now and have never had a problem, its not even much effort to close the pliers on the bars of normal hamster cages.)