I may be able to help. I'm the designer of NoCAN, a CANbus based IoT network (see: https://www.kickstarter.com/projects/1242572682/nocan-the-wired-iot-platform-for-makers).
In your case, I would suggest a design based on a MCP2551, which communicates with the Raspberry Pi with SPI. It's a popular approach, that simply needs to be adapted to the Pi Zero form factor. There are already Linux drivers for this chip so it's an easy path.
I can design it if you are interested. It might even be interesting to others in the community. Send me a PM to discuss details if interested.