How about a Basket? klick fix Rixen & Kaul, KLICKfix Adaptersysteme Fahrradzubehör, Solingen, Taschen, Körbe, Werkzeug
makes an adapter for seat posts
and the same basket works on it in the front, or on the back the interface is the same
just the one in back is made to go in the seat post , the front handlebar mount will let it go on the bars ..

Ditto handlebar bags ..

mainly a oval ring supports the basket or there are interesting bags that snap in place too ..

http://www.klickfix.com/images/downloads/bedext.pdf