two part polyfoam ..probably a 4lb foam would work. Mix the A and B sides very fast ( ratios are different with different foams ..i'd use a digital scale) pour in very, VERY small batches ( the expansion can be anywhere between 4 to 10 times). I repaeat the small batch portion of this message because it is safe to assume that this mask is out of the mold. Too much foam and it will fill the whole cavity of the mask ..then keep expanding outward, pushing against and deforming the mask. Small batches can be poured directly into the crown..then holding the mask upsidedown by the neck, slush the foam around the area you want filled until it starts to rise. When it does start to rise, DONT TOUCH IT!! ..it will collapse, turning into a hard mess. If you overfill a bit, you can always cut or pull some out when it fully cures. Mixing polyfoam can be tricky for a first timer. If this not something that you want to risk a mask on, simply cut upholstery foam up and glue it in with an industrial shoe cement such as Barge