RasmusB created a CAN Bus adapter for the Raspberry Pi:
This is an electrically isolated CANBUS adapter for your Raspberry Pi. Even if you screw up the connections somehow, nothing will be damaged. It also fits within the normal Raspberry Pi footprint, meaning that you can use it with most enclosures.
The board was designed with KiCAD and the design files are on GitHub:
RasmusB has made the board a Shared Project on OSH Park:
2 layer board of 1.58×1.08 inches (40.16×27.46 mm).