I saw the recent thread using handlebar clamps but Id always be worried they might fail under prolonged stress, so I bought an alloy accessory bar form SJS Cycles. This weighs around 110g and usually attaches to your steerer above your stem to carry your handlebar furnishings. I put it under the stem and inverted it to clear the bars and give room for the cables.
I finally gave it a proper thrashing yesterday - 25 miles of miserable muddy Chilterns bridleways loaded with a 13L Alpkit drybar with my bedroll stuffed inside it and it works like a bought one. Very stable - even more stable than hanging it from the bars though I'm not sure why - and the cables are free and clear to do their thing. As a bonus the bag is lower so has less effect on handling and no longer gets in the way of my lights. So Im well chuffed.
You need at least 27mm of vacant steerer space to fit one and if you have small frame or use a large drybag you might find front wheel clearance an issue. It fits very well on my large 26er Inbred.
