I think a key aspect to make it stay on would be to basically make it like a set of helmet straps. One around the head like the zeo, a chin strap and a top ver

I think a key aspect to make it stay on would be to basically make it like a set of helmet straps. One around the head like the zeo, a chin strap and a top vertical strap across the head. A headstrap works when your awake, but when your asleep you do actions like mash your forehead into a pillow and move your head up or down, which was a perfect action to remove the zeo.