Fitting the sensors to the bar is not hard. It's ensuring that they are in a correct and functioning position is where it gets tricky.
The reverse sensors have to be a certain distance away from the body of the vehicle. If they are not the spare wheel on the door and the towball sets them off. This is why if you look at the plastic bar it is molded out where the sensors are located.
The steel bar linked above is approximately 100mm too close to the body for the sensors to function.

