I took an armband holder for my Droid and attached it around a top tube bag. Its a separate little bag that works like a seat bag, and also makes the top tube about the circumference of an arm. Stuffed the top tube bag with a washcloth to make it stable. Then you can strap a neoprene armband holder around it nice and tight. It stays put beautifully, and still is lower in elevation than my handlebars so it should stay protected in a fall. Full setup was less than 25 bucks.
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