I have developed a custom I2C driver for my sensor. I used the Airspeed sensor as my example, stripping out the parts that I didn’t need. (calibration, etc.) If you search the forums, I think folks have asked about this in the past, it might (or might not) be helpful.
https://discuss.ardupilot.org/t/incorporating-external-sensors-to-pixhawk-via-i2c/24161/3?u=hunt0r