How to mount Galaxy S3 on a mountain bike
You're going to have to go with something purpose made for that specific model of phone to get a secure mount on the bike. I'm not aware of any universal phone mount options that are worth anything, however you can browse Wally World if you feel like wagering $20 one yourself. There are few different brands, but the Bracketron universal mount has sealed/waterproof zippers whereas the Arkon does not. I can tell you from personal experience that the Arkon universal car mount I have has had to be Frankensteined back into working condition about 4 different times over the last several years, so I don't have a lot of faith in their products.