You'll need to ensure there are supports around the inner edges. I set the support resolution to 2,00mm in Simplify3D to ensure supports are automatically generated. Check the settings in your slicer of choice to configure the proper support generation.
Be sure to print the case horizontally, that way you won't need a lot of supports. You can hit the shortcut CTRL+L in Simplify3D to select a face that should align with the printer bed.