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Mine are generally pretty snug - admittedly my design, so tweaked by definition for my settings. If you go to the last tab, Phone Dimensions, in customizer, you can try lowering the tolerances (5 Perimeter Tolerance and 5 Height Tolerance, if printing for an iphone 5) or even lowering the phone dimensions a bit. The Case Taper variable might also be useful. It defaults to 3 degrees, but if you increase the taper angle, it will pull the top of the case a little closer, potentially making a more secure fit.

Good luck!