How can I make a test bench for an electric motor?
My favourite is to use a second identical motor, directly coupled to it, as a generator. Then the generator is loaded with resistors for braking.
Often the motors will have symmetrical mounting holes, which allows the two bodies to be connected to each other with standoffs. If the motors are simply cylindrical, then I use two or three sections of angle, arranged round the two motors with strong elastic bands or tube clamps.