There is not enough air cooling on china dolls when stopped to really worry about the sides here. While forced air cooling like on a utility/industrial engine would be ideal, as long as the material isn't burning it will be fine.
If designed correctly the fairing could funnel air over the engine aiding in cooling at any speed over 10 mph.
The CAD mock up looks good and the simple geometry is a smart choice. As for actual constructing of the fairing: those eva foam jigsaw mats are cheap, light and easy to work with.
You may need some plates to sandwich the foam between to prevent the bolt from tearing through but something like this would allow you fasten to the frame without drilling into or welding onto it.