After a couple of bad experiences with trying to jerry rig my phone to my bike, I have finally decided I need a frame bag after my phone slipped loose going down a hill at 20 mph yesterday. Can anyone recommend me a frame bag for about $20 that is well built and secure? Also, this is for mountain biking, so it needs to be able to withstand all of the shocks and jolts from the bike bouncing around. Thanks in advance.