Researchers often use goat skulls.

I too would disagree with the 'object's surface area equal to hemoraghe area' and would say it would more approximate the dislodged area.

One often suggested object is the maglite but another one that is often suggested is a collapsible baton such as used by police and which would be quite possibly posessed by someone who was also enamored of stun guns. Such collapsible batons usually look like thick radio airials when the baton has been flipped so as to extend it to its full length. The extreme end of the baton would quite possibly be of the appropriate dimensions to inflict the wound described in the autopsy report.

All of this is indeed speculative, there is very little research on human skulls for obvious reasons. (Those darned bioethicists again)! And much of the research that does exist is patently absurd but oft quoted in various child abuse/shaken-baby syndrome cases.