Since you mentioned that you have a Garmin Edge 520, this will be your best bet for indoor training. It's what I did.
1. Go to your local dollar/thrift store and get a cheap iPhone case.
2. Get the SRAM Quickview Garmin Mount and sand/cut off the two clips on the top.
http://www.amazon.com/Quickview-Garm.../dp/B00DPUWXYI
3. Adhere the mount to the back of the phone case. I used countersunk screws and placed a piece of tape over the screws so that the phone does not get scratched. You can probably use some kind of epoxy/glue since you are training indoors, and won't be riding over any bumpy roads.