Do you need a curve in the slot if you use a double action truss rod?
Or perhaps, I should ask, is there a reason why Mark used a single action truss rod in the one piece neck instead of a double action one?

The two way truss rod goes into a flat slot in the front. It is hidden by the fretboard. So you need a fretboard...but it is easier but more expensive!

With the skunk stripe the slot goes in from the back so no separate fretboard is needed. The two way doesn't fit from the back - at least I have not found a way to do it...

Hope that helps!