In my searches online for an off-the-shelf solution, I did find these >
http://www.tourmaster.nl/en/11179339/vespa-mp3
No affiliation with them whatsoever. I was going to contact them to ask if they made one specifically for the 400, as that is the one that interests me the most for now, but if you do contact them first, do share.
Perhaps others with a keaner eye can determine the model/CC shown in their photos.