Best way to recognise flat circular objects?

Is there a way to recognise flat circular objects? Like a cyclinder but very short in height? Let's say like a hockey puck, or even flatter?  When I try to resize the cylinder in the demo you cannot make it flat, the whole thing seems to scale with it. Should I use an image target or a cylinder target? If I use an image target how do I deal with the "mask" around my circular image (given that the image is square) ? Is there a special colour you can use to ignore parts of an image or a way to create a mask somehow ? I read you cannot use transparency which is a great shame.

Cheers, I am loving Vuforia!

Gaz