Neck flexion and extension
If you have access to a neck machine, take the bad boy three to four times a week and work every nook for sets of 15-20. The problem is that most gyms do not have them anymore. Do not worry, it's a simple solution.
Neck elevations with a plate on the forehead (neck flexion) are the absolute money to build a thick neck.
I did this every other day for several sets of 20 pieces with a standard 45-pound plate on my head, but I worked three plates on my forehead for replays because I was a meathead. You do not need to do that. it really is not necessary.
For the neck extension work, I used a lightweight resistance band because I hate keeping the plate at the back of my head. Stay with sets of 20 or more.
And no, I do not like the neck harness. Making neck extensions with a billion pounds is a bad idea.
Construction of the complete yoke
Building a strong neck