I have several skulls hanging on my barn. A Buffalo, cow, fox, deer pig and turtles. Most of them were found in the field above ground, and the problem, like with the pig was getting the tough skin off. It hardened before the insects could eat it. Removal of the skin really helps. You might try what the taxidermist did with my Buffalo, he put it in a bin with maggots, he used special ones that leave the skull white but regular maggots will do a good job.