This project is super simple and much cheaper than something you buy in a store or online.
1/4'' nut (20 coarse)
Glue (try Loctite original Stick'n Seal)
Step 1: Obtain a Suitable Holder for your (camera) phone
I already had a great holder for my iPhone that I used to use in my car. I would love to recommend the exact same one. I got it from Walmart several years ago for about $7-8. I found what looks to be the same model on Amazon - (check the link or search "s type phone mount". I like this specific model because an ipod dock connecter fits perfectly into the bottom, so it can charge on the holder and makes the phone stay more securely in place. Also, if you bend the arms as shown in the pictures, it holds the iPhone perfectly without obstructing the screen.
Step 2: Get one nut (1/4'' - 20 coarse).
To be sure you have the correct threading, go to a hardware store and bring a tripod with you. While in the store, screw the nut onto the tripod to make sure it fits correctly. I believe all tripods use the same type threading. If memory serves, it was a 1/4'' - 20 coarse.
Step 3: Glue the holder and the nut.
The Loctite glue directions say to put some glue on both parts, wait five minutes and then stick them together. Once they're together, clamp & wait for 2 hrs (24 for full strength).
I ended up having to put braces on the sides to increase the strength of the structure. Also, I ended up using Super Glue for those braces.