Or what about instead. A Ferplast Criceti 100 - and put a door on the front
The whole top already comes off. As it's so hard to find cages bigger than 100cm I had been remembering the Carlos cage (two 98cm cages on top of each other basically - connected). And it was while looking for that I came across the perspex panels for the Plaza. Sadly you can only get the Carlos cage in Germany now - they won't ship to the Uk either.
Actually I quite like the idea of the Criceti 100 with a door put in the front. It's not a 120cm cage - it's actually only 95cm - but it's quite deep at 57cm deep! And 50 high. Not cheap though.
Ferplast Criceti 100 Hamster Cage