Carefully back fill the impression with hot melt (glue gun) glue. Try to keep it in the area and try to make sure it is flat and even. Use the hot melt all in one go if possible so that there are no ridges and bumps on the surface of the glue.
Anyone who has followed my other instructables will know that I love the glue gun. As an invention I rank it second only to the printing press. The use of the glue gun isn't mandatory in this project, but it sure does speed things up.
You will need a reasonable amount of glue (one stick) depending on the size of your impression. Of course you can easily use two part epoxy resin glue or any glue that is fluid before it sets, sets hard and doesn't really shrink. Hot melt is perfect for this job, because it flows very well when it is in its melted state and is flexible and tough when set and only takes a few minutes to set.
BTW... Elmer's Glue (PVA) is not really cut out for this job and if you use that you will be very disappointed and have to wait weeks for it to dry.