Let me to explain motor mount. This part is made with strong material, in this case with aluminum. This mount is small and not heavy, and very good fit under bike chair. In body of this mount is space where is added ESC Controller and Arduino. In the end of the mount is brushless motor who falling on wheel. In this 3D model in cap of this body is added also mount for battery.

Here is after 3D printed model of picture above, inside is enough space for ESC-Controller and PCB.

On top of cap is also mount for battery, in this case is mount for Makita battery, with that case you can easy put and take battery.

On image you can see how all that looks good when is included aluminum. Is not same model like on 3D model but same works good.