Building a low-cost small-size CAN-to-I²C bridge
There are LPC11C00 microcontrollers with integrated CAN transceiver. Should be trivial making a board without isolation. But these parts are only active until 2020.

But you could also get a small STM Nucleo board (eg: NUCLEO-F042K6) and put one small custom board with the CAN transceiver underneath.


  but I would like the hardware to be open 


You can make the board open yourself. Contribute.