... however:
I'm not a lawyer or an insurance agent but if you build something that you "hook-up" to any device that has regulatory compliance requirements; you may be breaking laws and may find yourself in serious trouble should a mishap occur.
Ex 1: Friends attach your device and take the automobile for a spin on freeway in heavy traffic. The automobile stalls and the car behind crashes into them. The car is impounded while the State/Feds attempt to discover what caused the engine go turn off. The rear car driver hires an attorney to recover pain and suffering and the fact that an unauthorized device was connected to the CAN bus becomes an item of importance.
Ex 2: Friends attach your device and go for a ride. All is fun until the police pulls ghem over for distracted driving. Seeing the home-made device, the policeman believes it to be a timer to blow up the fuel tanks. Your friends are arrested as possible terriorists. Things go downhill fast....
Ray