This is just an encrypted disk image, while FileVault is much more than that. The big thing missing from this is the recoverability with your MasterPassword. If you choose not to remember the password for the encrypted disk image in your keychain, and later forget the password, you are out of luck. Also, as mentioned elsewhere, mounting in /Volumes is not the best idea.
To fix this, forget DiskUtility, and behold the power of hdiutil!
The man page for hdiutil is quite a read, but it outlines the options that will allow you to wrap the image with your filevault certificate (-certificate), use a sparse disk image (-type SPARSE), use encryption (-encryption), set the default permissions (-mode, -uid, -gid) and more.
Hdiutil also lets you mount the finished volume somehwere other than /Volumes using a command like: "hdiutil attach ~/mydisk.sparseimage -mountpoint ~/Encrypted"
That command can be wrapped up in a double-clickable applescript to make mounting an easy process.


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--DH