Head straps are in most boxing, MMA and other sports raining centers. Leather, nylon or heavy cotton. Remember to use front & back positions when using one. Most just use fron tfacing positions.
I know Montague knows this well. Wrestler bridges are exceptional, though some have a fear of them..if done right, should be no problem. Also old school head lock drills are good.
Some of the best of results have been with a training partner giving resistance to the neck; hitting all three positions, front, rear and the sides. Can use a simple towel, strap, etc. One of the advantages is that negative reps can also be given, with the positive reps.
Shrugs, cleans, Hi-pulls, etc don't always give the max results if looking for a larger neck. Necks, on the average, tend to grow somewhat fast, when focusing on them in serious training. So many BB'ers have what is called pencil necks.
Good Luck