In the back of your manual (p216) it has the accessories relevant to your model. In case of the 550D it is the EP-EX15II (£18 in WE).
I have one that I no longer need, used it on my 450D. It gets your nose away from the screen Ok. You need to be a little bit careful when stowing your camera in a bag as it can get knocked. It also makes the scene in the viewfinder a little smaller, so that bit more difficult to focus manually. Having said that if you're having trouble it's a 2second job to remove it. All in all I was glad I got mine and used it all the time.
Make me an offer. After all it's still got most of its good photos left inside it!
Cheers, Sie