sorry for the confusion.
if yours isn't the one with the barn doors in the back, you could just use some plexiglass and bend your own, use some double sided tape to attach it to the glass and call it a day.
though this isn't a great example, here is one on a metro (but made of coroplast). it is similar to what I am talking about.
if you used plexiglass and used a heat gun to bend it into shape, you could make a similar design and it should be a lot sturdier and a little more eye pleasing.
you could make it out of these materials to start with as they are cheaper (sometimes free) and you could get an idea of the angle and what not.
it does do more with higher speed. the aerodynamics become a large factor at speeds over 55 (as a rule of thumb)