I use a Carradice bar bag. It has a light but strong corrugated plastic sheet box covered in canvas with a Rixen and Kaul Klickfix system bracket.
R&K and the similar Ortleib bracket is by far the best. It relies on a wrap-around plastic-coated cable for support rather than clamping lots of plastic onto the bar. As a result the bar clamp is quite minimal but totally secure even down rough tracks. I load mine up with guide books, camera, bits and bobs. The map holder is quite useful, speeding up navigation in areas of complex road networks.