This looks like what you're after?
http://www.ebay.co.uk/itm/PANORAMIC-WIDE-REAR-VIEW-MIRROR-ELASTICATED-STRAP-FIXING-/191258582424?hash=item2c87e65d98:g:4ZUAAOSwk5FUxh5j
Out of interest, do you find it a significant improvement using one of these? Are there any drawbacks - vibration, movement etc?