I would cover a cake board in black/grey fondant for the brim. Then use either a 1/2 ball mold (if the appropriate size) or stack the size cakes you want and carve a rounded top to them (I hope that makes sense). You can carve slight indents for the detailing on the hat (where it looks pushed in). Cover with the same fondant as on the cake board/brim.
Hope that helps or at least sparks some ideas for you.
AGREAT idea on the cake board for the brim! Why didn't I think of that? Haha. Thank you so very much! :)