I wanted to keep the base of the 3D print flat (for better quality) rather than printing sides and using supports. So, I printed it flat and used Sugru to build the walls.
Step 1: Print a Platform
It's here: http://www.thingiverse.com/thing:126372
Step 2: Make the Corners
Create a little ball in your fingers.
Apply it to the corners and shape it into holders.
1. Make sure it overlaps the bottom.
2. Make the edges fairly high so the iPhone can't slide off.
3. Use Sugru of the same color as the print. (I'm using contrasting colors for this instructable.)
4. After applying, stroke the Sugru lightly with your finger tip to smooth it out.
Step 3: Make the Sides
Roll the remaining Sugru into a long snake.
Cut it in half.
Press each strand onto the edges.
1. Leave the short sides Sugru-free. This will allow room for the charging cable.
2. Push the Sugru so most of it is on the top of the platform.
That's all! Let it cure overnight and you've got a great iPhone platform.
Thanks to Instructables for providing Hoboken MakerBar some Sugru for a Sugru Hack Night.