well, you got 1 of 3 options:
1. You could get the faceshield, and its flexible and is alot easier to install and is about 7 dollars w/o shipping online.
2. You can take a dremel or razor blade, and trim down the inside where the visor will sit so it will be more flush.
3. Put the acrylic in the oven at about 250* and let it get soft and flexible again, then when it is flemsy, pull it out, and place it inside over the visor area, and hold it so it cools and hardens and forms to the visor more flush.**
**Now by doing that, while the acrylic is soft, it is prone to scratch very easily. so if you dont feel comfortable trying it you might not want to, unless you have access to getting some more acrylic.**