I have a 6 and 8 inch half ball pans..Use something like that for the hat and make a brim out of gumpaste...Acyually very easy to do...
I used the ball pan as well and then took one of my son's new baseball hats and wrapped it in press and seal or saran wrap would work as well. Used the brim as a model and covered it in fondant and let dry. I used a Wilton cutter tht has the interchangeable parts, for the lines on the top part of the hat...actually had a wheel that worked well. There are many tutorials online that will help you. I had never done one before so I was suprised it turned out so great. Traced the logo from pic of my nephews hat to cut out in fondant. Good luck, it will be great.
I just made one a couple weekends ago, and carved it out of two stacked 8" round pans.
Here's how it turned out with that method: http://cakecentral.com/gallery/2147196
In hindsight, I should have made the brim bigger. Guess I wasn't planning on the round part of the hat actually being as big as it was 
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