Improved Garmin Oregon 450 bike handlebar holder

This handlebar holder is slightly improved (original: http://www.thingiverse.com/thing:24021). Due to some broken holders, I've added some material to the edge between handlebar mounting and the small release lever. Furthermore, I've separated the upper and lower part of the holder in order to print them independently. The lower part was rotated such that it can printed without large overhanging sections.