The rear crest is flat since I started this model. Today I reshaped it to make it three-dimensional. I also added an extra layer of foam to the orbits, because just molding the foam was not enough.
First I removed the layer of acrylic resin around the orbits.
Then I added a sheet of foam in approximately the same shape.
Then I did some carving, molding and a bit of burning.
For the rear crest, I added an extra bone on each side, and then filled in the spaces with pieces of foam making it slightly wider near the middle, but narrower below and very narrow above (where there would be soft tissue). Here are some pictures. This is a side view.
A view from the far end of the rear crest.
A view from above.
And a view from below.
The next step is to add a layer of acrylic resin (and some gauze, if necessary) to fill in the gaps, sand it a bit, add some stains and colors and finally declare this skull finished.