Before automotive air conditioning became affordable and popular, there were a variety of ways to help keep your car's interior from becoming unbearably hot on sunny days. One of these was an external sun visor, often available as a factory option (also as a dealer option, for cars that didn't offer that luxury from the factory).

Most were metal, but some were translucent plexiglass

Link to example:
http://www.pontiacsafari.com/L1Garage/WindshieldVisor/index.htm


ETA: I have occasionally used one of the stand-off bumpers, but the sharp corners that result don't look authentic. Something with brackets at the extreme ends and a center prop would create the effect well-enough.

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2576835865