I'm a HUGE reggae fun, but I don't have a hat to loan you unfortunately. Don't know how much it would cost to mail one back to someone but here is a link for one halfway cheap and it would be yours. http://shop.store.yahoo.com/spencergifts/407833.html
To be authentic just make sure you smoke a big fat spliff and reek of ganja when you roll into work! Sure your boss would love that!
Seriously, good luck with your costume and have fun with it!
Cool Runnings Mon and keep that Irie feeling !