This is a simple addon for motor mounts in the base in order to increase the rigidity and to prevent the mount from being bent by the belt tension. Its features include:

1. simply print and insert, and that's it (see pics); no need for assembly or removal of any existing parts in the stock setup (among the pictures, two CAD drawings show how this add-on, the wireframe, is positioned with the stock motor cover, the solid drawing, where the light orange colour show the contact surface with the motor, indicating how this addon works mechanically together with the stock cover);
2. the rigidity it provides is among the best in similar designs, because the torque it generates against the bending is the largest possible given the confined space inside the base;
3. it serves as a partial damper for the motor vibration; some other full-sized designs may have direct contacts with both the motor and base wall so that vibration of the motor is more easily transferred and amplified (and thus larger noise) through the full-sized design;
4. it has better durability than some full-sized designs; those designs do not contact the base wall and thus relies on the rigidity of the 3D print, so the user need to be cautious about it's durability because most 3D printing materials are environmentally friendly and thus could deform or even dissolve over time especially under force, as constantly experienced in those full-sized designs (you could survey for the proper materials if you'd like to use the full-sized design rather than the addon here, which a PLA is good enough for);
5. you could insert a thin rubber pad between this addon and the existing motor mount if you want (if your rubber pad is thick then you could simply cut out this design before printing).

The white thing at the contact between this addon and the base wall in the picture is bluetack (in white colour). Alternatively you could use glue gun and or or silicon sealant. I was just too lazy but it works.

Let me know what you think! Have fun.