First, how much does the visor still need to curve to match the shape of the helmet?
Second, how much flex does the visor have? ( so long as your visor isn't made of acrylic, it should be fairly flexible )
If your visor has plenty of flex, and it is only maybe a centimeter or two off of the contour of the helmet, then leave it be. Once you have the chicago screws glued in, all you have to do is gently flex the temples of the visor to fit it in. The added pressure of the visor pushing outwards against the helmet will help to hold it in place. 