I don't use a handlebar bag, but do use this for a map. It's about 6X6, and just attaches w/velcro on the bars, and rests on the stem. You can flip it up to check out the other side. The only thing about it I don't like is that if I stand to go up a hill (which I don't do that often, being a sitting slug), it will hit my knees. Not a big problem, I just fold it under until I need it.

http://www.campmor.com/webapp/wcs/st...berId=12500226