/cgl/ - Cosplay & EGL

I need to make a top hat that is larger at the top of it and the size of my head on the buttom part (see pic). Is there any way, mathematically, that I could figure out what kind of shape to draw out on a large sheet of craft foam + wonderflex (Only materials I have access to atm, tight time frame as well)

So, essentially, I need to make a cylinder with the size of my head on one end, and then it gets larger on the top end, preferably as one shape. How do I even