How can I change a double DIN unit to a single Din unit in a Vauxhall Meriva?

I have a 2004 Vauxhall Meriva with a double DIN tape player. It's pretty basic. I would like to change it to a CD Player which supports mp3's. The equivalent double DIN CD Players are quite expensive. I have seen some good single ones but how do I fit it into a double DIN unit?|||You will need a dual to single din mounting kit. Then just about any deck will do.