Very nice looking undertray. If you drive higher speeds you might need some more connecting points. Specially in the front I needed to add a rigid bar to stiffen the panel. It started to hit ground first at 140 km/h speed and after that also at lower speeds. So it broke the panel a little bit.
Did you make a hole for the diesel cooler? If you have one there under the car you need to make a hole. Diesel will otherwise get so hot that the plastic tank will melt or that is at least a risk.
Tank venting is this piece what I took out:
You will get 10 liters of more fuel when you press that nibble when filling.