This.
Another exercise, well an old school one my coach told me, is to lay on a bench, bed, or anything where just your head is hanging off, raise your head forward like you're nodding, look to the left, then look to the right, then return your head to the previous position. don't jerk your shyt too much though cuz you'll injure your neck but do it quickly, that's one rep. Repeat until failure.
If you grapple, then grappling itself will also build your neck, especially clinch work.