so basically ya use reverse wedges. made of the same wood yer plaque will be made from. if yer real good with wood, ya make 2 half circle wedges that make a nice single circle when put to gether..then ya have to make the inside edge, this is where the bases of your skull will go when ya put the two halves together. it will take sum tweaking cause the skull isnt the same all the way around? it needs to be tight. it shouldnt wabble, so cut small.. make sure ya screw it all together from the back side.

if ya aint so good, ya make 4 er so, and strategically place them, 4 corners if you will, to hold the skull to the board (plaque).