It's pretty simple from python, but it will require the ctypes library so you'll have to install python to get it to work.
Add a Window object to your cap and run this at the start of the layout:
from ctypes import *
hwnd=Window.AppWindow
region=windll.gdi32.CreateEllipticRgn(0,0,640,480)
windll.user32.SetWindowRgn(hwnd,region,0)[/code:36plvr02]