If you are thinking of making this case work with a z1s, I'll mention this: The t-mobile z1s would probably not work with this z1 case, even with careful mods. Some of the cases made with flexible materials can stretch to fit the z1s, but there are significant port location differences that would probably require a model-specific version of this case to make it work. The overall dimensions of the two phones are also different enough to make this type of zero tolerance case not work.
The z1s headphone jack on the top is in a different place. The rear microphone is also in a different place, (relatively easy to drill out two extra holes)
The LEFT side is the hardest modification to make. (I have tried to modify two stretchy z1 cases) On the z1s, the USB port is almost at the top corner of the phone. Re-cutting an additional opening for it on a standard z1 case weakens the case considerably along the bezel side. (There will be a useless pre-cut opening above the magnetic charger area, and the top corner for the new usb flap needs to be very close to the top corner.
I modded two z1 cases that do work, but now use the t-mobile body glove case, because it just works better. I doubt that this one could be made to fit.