I took a glossy version of the Ubuntu Studio logo and rotated it by 13 degrees so that no one side is obviously... predominant? I then downsized it to 24x24 for use as a regular old Gnome menu icon. It does have an alpha channel so don't pay attention to the black background in the preview.
I use UbuntuTweak to replace the icon so I am uncertain where the file is actually located that you will need to replace but I'm sure Google knows. ;)