Jen, kyrill makes some good points. If your dwarf hamster has always lived in a Mini Duna by Ferplast, the next step up would be the Ferplast Duna Multi. So if you did choose to upgrade, this cage would look and feel the same to your hamster as it has the clear upper lid and safe narrow barred grill. You could then place the plastic shelf from the mini Duna inside this larger cage (the slits on the shelf would slide between the two parts) and balance the other end with a bendy bridge or something else that could safely hold it up.
There is another site that sells this cage, I will post the link shortly. x
Edited to add:
http://www.portonaquapet.co.uk
It`s sold on here for £33.69p