Buck converters and fusing
Finally found the link for the ones we got. Used for both running 12V fans on our 3D printer, replacing the noisy 24 vdc OEM ones. And for providing 5 vdc to run my RPi 3B from 12 vdc battery. Worked well for us.
Printed case to hold them.
And fuse holder to protect the RPi in case of a problem.
Lots of other choices available depending on what your interests and pieces / parts happen to be.