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TN_Mike carries his Glock when biking all the time, but in his case, open carry is an option. With the amount of movement and sweat generated, I'd shy away from leather, and lean toward kydex, and possibly a retention holster.
Blackhawk makes a concealment kydex holster with their Serpa lock, you might want to look into it. As far as cover garments go, a light windbreaker shell or a longer t-shirt would probably work. You're not breaking any land speed records on a mountain bike.