That's very similar to what I've done, but as a sidenote: part of DDG's appeal is its lack of tracking. Therefore, it might also be wise to change the search URL to "https" instead of "http" to ensure that each request is encrypted. It might take longer to process because of the SSL handshaking, but that's an inconvenience I (and probably many others) are willing to take.

The other advantage to this is that when searching from the included "Search" app, your DDG search now shows up as an option, which is great for initiating other things using it's !bangs, such as Wikipedia (!w), IMDB (!imdb), Amazon (!a), or even Google (!g).