As far as the boards are concerned, it's not really that hard. Basically, you could build the whole thing on a perfboard. But who owns an etching machine his own certainly has a small advantage.
Connection Diagram for the Pressure-Sensor and Display
2CASE and Slider STL Files
You may have to make one or the other change to your 3D printer.
Of course, the result will be even better if you have a SLS printer or you are using a print service, such as Shapeways.
3Getting the source code
Download the source from Github.
To compile, you will need the i2c_t3 library instead of the standard wire library.
You'll find it at: https://github.com/nox771/i2c_t3
You also need this library :
Adafruit GFX - https://github.com/adafruit/Adafruit-GFX-Library
Ignore the .vcxproj and .sln - Files. I use Visual Studio together with Visual Micro to develop, because it's more convenient.
If you install the software for the first time on the Teensy, in setup() you should comment in this line:
After flashing you have to comment out this line and you have to flash the teensy again.