It’s kind of scary to cut a big hole in the helmet, but it has to be done. I’ve looked over the engineering drawing for the MK V helmet, and tried to measure as accurately as possible where the viewport opening should go. And I’ve adjusted the location just a little, by eye. I’m using an inexpensive hole saw, from a kit purchased at Harbor Freight. The quality is not as good as something you’ll find at a tool store, but for limited use it should do the job just fine, and save a lot of money.
I’m drilling a pilot hole to get things started. Hole saws can be very “grabby”, so I’m moving slowly so that the saw doesn’t throw the helmet across the room.