Your car should already have one to prevent bugs at 100+ mph hitting
your eyes.  If not, get one from a windshield shop now!  :)

If you mean a windscreen at the back, popularly known as a windstop,
I have found my Oris very effective at stopping wind and good looking
since 1997 or so.  I usually keep it below the folded top, (saving
trunk space), and take it out as heat or cold or unavoidable
Interstates dictate.  As soon as I am off the Interstate, I reach
back and knock it down.

The Oris is very effective, though there might be stops that block
the air even better; I remember one Plexiglas one wrapping around to
the side windows.  But since no stop can be as effective as raising
the top anyway, I never got excited about any of those.  The Oris
looks best in my opinion and can be pushed down when the need ends.
Also, the Plexiglas ones might be very hard to store.

I replaced the Oris bottom straps that wore out with Velcro.
I also had a Plexiglas stop, but did not like the wind buffeting
and reflections.  The Oris does reduce contrast behind you; I find
it hard to distinguish police cars through it.

Leon