the top curve is a 15 inch diameter on the one I printed. This can vary from print to print and depending on how you glued it together. I haven't done the visor part quite yet. Best bet is a round can and a heat gun. Once you get the basic shape, you can use the heat gun to get it all exact shape. then trace the outline of the opening with a marker of some kind. When you cut the opening, make sure you leave enough "plastic/material" so you can sand it down to fit perfectly. I'm still working on fine tuning the design and sanding mine smooth so I can do molds. I will upload a template of the visor that can be printed in an hour. Thats the one thing I forgot.