Of note KLICKfix Vario Rack which uses a bracket which you may fit to the BF steerer stem and carry what waterproof Rucksck you may own
or a River-portage pack or Dry-bag.. http://www.klickfix.de/ see new products ...
anyhow .. will be interesting to see from someone who bought one,
whether it is different from the rear rack mounted
Ortlieb Downtown bag which has specs of being:
36/14.2'' x 36cm/14.2'' x 14cm/5.5'' = 18L/1098cuin.
leveling the rack mount could end up with something that you could hang
from the top rail of a porteur front rack
Brompton's front rack for M and P bar bikes is 40cm x 30cm,
(the base of my Brompton 09 Touring bag is 20cm deep)
Todd at Clever says O bag 'per the site: 20 litres capacity.
50cm x 30cm x 15 cm high' [must be laying down,high = depth..]
standard frame 40 x 30, so hare wider , bit shallower..