Aerodynamics is a rather esoteric subject.
At a rudimentary level its pretty much common sense. Very simplisticly, at the speed ranges involved on a bicycle two factors will come to play a part. Shape or frontal area, and thus the volume of air displaced. The biggest factor will be drag which can be mitigated a great deal by simple design. A rough edge that meets the airflow at an acute angle will produce enormous amounts of drag, whereas a smooth rounded shape will discourage the smooth airflow from seperating from the surface over which it is passing providing far less drag and turbulence. Some basics here,
Beginner's Guide to Aerodynamics
In this application, it is the drag induced by the passage of the foot shield through the air that will affect its drag and the level of turbulence. As speed increases these factors can increase dramatically. Quite often, if it looks smooth and 'Aerodynamic', then it probably is.
I would suggest the best clues will be found on motorcycle fairings as they need to deflect the air without generating turbulence that would try to remove the rider's head at speed.
A difficult subject to summarise in a few words but I've been interested in the science of aerodynamics for many years.
I think it might be easier to buy some warm socks and boots.