See if any local welding supply stores have #14 or higher glass (or helmets) in stock. Some auto-shades can go that high, but many cap out at #13, and they're generally going to be >$300 for the ones that can darken to $14 or higher. If you end up using an auto-shade helmet you'll have to set the sensitivity and delay to their maximum values. Don't buy an auto-shade helmet that doesn't have these controls. It's also possible to stack a #10 welders glass plate inside an auto-shade helmet and tune the shade value to get to #14 if the auto-shade can't get that dark on its own. It should be impossible to see anything but the sun through the helmet + glass combination.