Does anyone have an idea on how to make a grad hat to sit on top of a cake. I want to use a large cupcake base for it, but I am having trouble with the top

I don't know how big you wanting it to be. However, I am planning on using half of the sports ball pan from Wilton for a cake I am doing in a few days. I used it a couple of weeks ago to do a Minnie Mouse hat with ears and it worked great!

Here is what the half sport ball ended up looking like.

http://www.cakecentral.com/cake-photo_1675946.html