Position of object for moulding/casting

I would do a 3 part mold with the top of the hat facing down. Pour the first half up to where the hat band is. Pour the second part inside the hat and the third part being the hat band and brim.

I personally would not worry about the angle of the pour ,rather the gates that will channel the resin into the cavity. When making the mold, a slight tilt of the hat will make for a better draft angle than if the hat was just sat on its top. The slight tilt will allow air to escape easier verses it being strait up. Try and put the pour gate somewhere inconspicuous so that when you clean up the excess material it wont be so apt to show. Make sure that you have a vent gate(s) for the excess air to escape from...this is important as any trapped air will cases unsightly bubbles ,or as in your case, voids in your finished product.
What ever Alumilite product you use, be sure and fallow the instructions. Measuring ratios are critical to fallow. Alumilites website has wealth of information available.