My smart phone VR Viewer that I bought on eBay has a cavity that fits dimensions up to 14mm x 80mm x 150mm - and employs suction cups to 'suspend' phones smaller than that dimension in the center of the viewing area. They don't work... at all.

So, I made this bracket which holds my iPhone5s perfectly centered, with or without it's sleeve on!

The two detents were added so that the volume buttons wouldn't get activated while in use.

Enjoy!

Feel free to check out my other designs which are as diverse as they are many:

http://www.thingiverse.com/3E8/designs