I also made a direct drive, but did this by drilling (turning) a hole in one x and one y axis, and glueing the motor shaft in.
So what you get then, looks like a stepper motor with an enourmous shaft.
This motor + axis can then be mounted to the UM, without the coupling, and without the extra added length.
Watch out: Because especially the motor shaft will never by exactly straigt, so rigidly mounting the motor to the UM-frame is no good idea. Therefore you will need what we call a torsion arm to transfer the driving torque from motor to frame.The torsion arm should be stiff in rotational direction, but flexible in the other directions, in order to allow the wobbling of the motor. I made two torsion arms from 1.5mm thick aluminum plate, and connected both with 4 countersunk screws to the stepper motor. I put both motors on the left/back side. Will add a photo later.
I had to reverse the direction of x and y motor, otherwise the printing of multiple object went wrong. I did this by reversing the wires of one pole of each stepper motor. (in the connectors there is a little catch for every wire, you have to press it in, so you can swap wires without any soldering)
I did not measure the before/after accuracy, but the belts are definitely the most important compliance (1/stiffness), so it will improve accuracy. Because friction in the xy-mechanism is quite high, accuracy will mainly be determined by the so called virtual play: friction/stiffness. (Newton / Newton/mm = mm)
Main reason for this "hack" for me was that especially the short belts lost pretension very often, but the extra accuracy was another reason of course.