I rather like this from Carradice.
This is the long flap version and if necessary can carry large bulky loads by opening out the top flap by half as much again.
This one sits on a small support clamped to the rear stays. Lots of handy side pockets and space.

This is the long flap version and if necessary can carry large bulky loads by opening out the top flap by half as much again.
This one sits on a small support clamped to the rear stays. Lots of handy side pockets and space.