What you need to do is grab some E6 slide film, and then have it processed completely unexposed. You have to make sure the developers are aware that you know it's unexposed, and want it developed anyway, and that it needs to be uncut. The blank and black processed film will block visible light, but not IR. So once that's ready, all you have to do is cut it to size, and layer two pieces on top of each other. Once you strap that over the flash, when it fires, it won't send out any light that can disrupt your image, but the slaved strobes will register it, and fire accordingly.