get an ebony sort of dowell rod, shove that bitch in there and drill it out.
it'll look more like a fender.
wait, upon further inspection it looks like its cut like a gibby truss rod slot.
is this tru?
where did you score that neck btw?
maple and rosewood, i scored one of each, not bad for $125 each but the truss rod hole is UGLY. it looks like a smaller gibson style slot which means a truss rod cover, or finding a way to hide it.