There's the old Direct Connection ones that were make from ABS plastic. They look like this: http://www.moparmagazine.com/assets/images/sep-oct_2008/reader_rides/rr_7.jpg

I think MAS Fiberglass had/has one that was the same but just made from 'glass.

Then there are the ones made from aluminum that you see on Ebay a lot: http://spoilersbyrandy.com/dusterpictures.aspx . They are not as wide as the DC style ones and have curved ends.