I have been using a rego label tube on my bikes for some time now. You can see it is that that blue thing just above the pipe. Before you ask yes it's legal - How do I know- Well lets say it has been roadtested by the local law!
The inner tube part rotates so you can check the whole label.
It is always fairly easy to find a place to mount it as it doesn't take up much room- there is also a version with an LED light inside...
I got this one from austreetfighter.com.au. but you can get cheaper all plastic versions from bike stores for around $14
