Gren's trying to design his for a bit more durability, plus whether or not the mask works doesn't matter as much as if the machine providing the air does. He just needs the mask to work to keep water out of your mouth and the mechanical bits, so he can give it to say, Shiro, and see if the devise works properly. If it does he'll later make himself a set of "Aqua Lungs" for his body so he too can breath underwater without needing to strap on an air tank for himself. The important thing about his mask is that it's compact: So small that it only needs a mask, not extra tanks, no tubes. All he needs it to make sure water doesn't just go into your mouth or the mask by the time you're already underwater.