Help with Building "Extension" Bin Cage

Howdy!

Mo already has a standard wire cage from Pets at Home which she's had from day 1. I've decided to expand her empire, and after seeing some of the ideas on these forums have decided to go down the bin cage route.

My idea is to connect the current cage into the bin cage using some rotastak connectors (out of the side of the original cage, turn down and then into the bin cage below). Problem being I'm struggling to create a circular hole in the bin cage without splitting the whole side!! I'm guessing I'll have a similar problem when it comes to putting a hole in the existing cage.

Does anyone have any tips / ideas / advice on correct tools that might help me out?

Thanks.