How did you mount the skull on the board? I'm still trying to figure that out.
I'm sure Bparks will answer with his method, but there are several methods. The method I've come up with is ut use an 11" white cable tie (aka zip tie). There are three pairs of holes on the underside of the skull in the bran area. I pass the cable tie through one of the pairs of holes, depending on which pair makes the head "fit" the board best, then down thru a hole in the panel, then back up through another hole about 1" from the first hole, then zip it and draw it tight. I've used this method for years and have not had one failure. Since the head can move around a little even when drawn as tight as possible, i put a little tiny drop, or touch, of hot melt glue on each point where the head contacts the panel, meaning the two bones protruding down from the ear holes area and the second or third molar. I do this prior to attaching the skull, just to give it some traction so it doesn't slide around and scar the panel.
As I said, there are many ways to acomplish this. Below is a visual aid to help understand my method.