If I understand correctly, you are using a stepper motor to move the light, and you are looking for a more robust rotating mount which is simple and cheap.
First, I would reduce the length of the arms on the U bracket that the light is mounted on. Longer arms create more stress on the motor bearings.
Second, I would find a stepper motor with a longer shaft so that you can screw a collar onto it that will be attached to the U bracket. The collar would be attached to the shaft with a grub screw.
Third, make a bearing from a flat piece of nylon or teflon and a smooth flat steel plate that the bracket and collar will sit on. The bearing will go between the bracket and the top of the box. You may need to use a geared stepper motor to increase the torque.