This is a case I designed for the Google AIY Voice Kit Version 1 (*The one that uses a Raspberry Pi 3, not the Pi Zero*).
It is printed in 3 pieces that are securely held together with pegs, and is a bit more compact than the original cardboard case because it moves the speaker into a horizontal orientation.
All of the ports of your Raspberry Pi are accessable through gaps that should be wide enough for their respective connectors. This design does not use screws or extra hardware.
V2 Jun17 2018: Edited the pegs for the microphone and pi to have a thicker, sturdier base. Also revamped the holes for the hdmi, 3.5mm, and power, to be a single hole to better fit plugs of unusually large size.