If you've got a Dark I assume that you have no seat cowl - so a tail pack that sits on the pillion perch is the most economical option. Loads of companys make them... only downside is it makes getting yer leg over the bike a bit more difficult
See
http://ukmonster.co.uk/photopost/sho...00&ppuser=1208
If you need more space get a tank bag - Oxford Humpback is good here as it does not get in the way of the riding position leant forward over the tank, and it gives you somewhere to put map/route details.
Ventura system is good - but expensive.
Miles