I have used two part foam with no problem. I put a plastic bag from the grocery store in the mask and fill the bag in the mask with the foam so it won't stick to the mask in case you want to remove the foam later. When the two part foam is expanding you can push on the mask to shape it the way you want and make sure it doesn't look bloated. The two part foam will set in about 5 minutes so don't have to wait long. Great stuff can take for ever to cure if you don't do it a little at a time and do what Bubbles suggested about spraying it with water to force it to cure faster, that could work too. If you can get your hands on two part foam it works a lot better. Two pound should be fine and it will expand a lot more and be lighter.