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Use a kit. There's two types: The kind that puts a 1/4" of plastic above and below, and the kind that puts the deck on the top or bottom allowing you plug the gap with a pocket, eq, or just plastic. (some people also put their alarm lights, remote radar, etc. there.
The 1 1/2 din and double din radios cost more for less features. Often, they don't even have two sets of RCAs out the back for hooking up amps. Lastly, they are very difficult to transfer to a different car as most cars(maybe C6?) are single din. This also makes it much more difficult to sell if you want to change or upgrade later.