I worked on designing such a thing while at university using differential angular momentum - kind of like a cube with a high-speed rotating wheel embedded in ea

I worked on designing such a thing while at university using differential angular momentum - kind of like a cube with a high-speed rotating wheel embedded in each face. The wheels on opposite faces cancelled out each other's angular momentum - but if you vary the speed of one of the wheels slightly, it would give a small amount of angular momentum about any desired axis.