That mask is giant.I dislike the "one size fits all" motto they use to design these things.
One thing you can do is use Velcro to cinch the back.Make a slit and then apply the Velcro applying one piece a few inches away from the edge of one sides edge.You can probably get the hair to cover the slit.
Definitely try the foam or sponge idea. I used foam packing in a Darth Vader helmet to put my eyes at a certain level so that I could see better. I just used that sticky backed velco you can purchase at Wal-Mart for much of nothing to secure the foam padding. Worked great!
Yeah the foam idea is best. I live in Houston and beleie me even in October it's flat out hot outside. I use foam pieces to keep tight masks from pressing into my eye sockets, a couple of years ago I boght this little half skull mask with tons of hair and the eye holes pratactly popped my eyes out so I stuck a couple of foam applicators inside theask just over the sockets and it fit great.