I took another photo today, this shows the aluminum support I made much better than previous photo, it is two bars of aluminum, 1 1/2 inch by 1/8 inch. There is a bit of inner tube rubber sleeve on the end so that the aluminum does not chaff and abrade the bottom of the bag. When I hit a bump, it bounces a little bit, but it keeps it in position much better.
This was on an old Louis Garneau (spell?) bag, not water proof but I have a rain cover for it.
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If you are new to handlebar bags, they are great when used with a quick release bracket, I can keep all my valuables in it when touring so that I can carry it in restaurants and grocery stores with me, etc.