rout the whole thing to a constant depth so it's nice and clean, then cut a peice of wood like ebony, wenge, purple heart, anything really hard, and take your time, make sure it fits fairly snug, to the point where you ALMOST have to force it in... then put in some titebond, let set for a couple of days, then rout your truss rod again.....
so it just looks like a skunk stripe once you've contoured the back of the neck.