This is a basic iPhone 4/4s case that can be used as a basis for custom cases. This is the case that I used to make my Gear Case with Geneva Mechanism, and Card Holder Case.
This case is easy to print, strong, and a great fit. The areas beneath the buttons and sliders has been removed to make them easier to use, and a cutout has been made around the headphone port so bigger jacks will fit without having to remove the case. The edges of the case are also tall enough to prevent the screen from touching when placed face down. I added a bevel to the ledges that hold the case onto the phone that makes the case very easy to print without support.
Print the case with 0.1 mm layer heights, no support, and single perimeters. There is a slight bevel on the ledges that hold the case onto the phone that make it printable with no support. The bevel will only work if layer heights are low enough though. You may want to scale the case up by 1% to account for shrinking.