You blew it up (yes like a balloon or rather a summer plastic swimming tube for the kids, then fitted your flash, facing into it. There would be a thin plastic sheet acting as a diffuser between the flash head and the transparent balloon surface. There would be elatic rubber bands holding the flash to the balloon.
It still inflates. But the rubber bands are long gone. They did not last more than a few uses.
It sure was a conversation piece. And an icebreaker when used.
