For Halloween last year I decided to be Ash Ketchum because I had a giant Pikachu, but the hat wasn't in my reach so I decided to make my own.
I bought all the materials from Michaels for around $5
Paint all the parts of the cap except for the front red and let it dry
Google an image of Ash's cap, and sketch an outline of the green symbol in the middle of the front then paint it green.
Let dry and VOILA! :D