Hello. I made a simple head harness, and I think if you are tight on money like I am then you can follow my simple design.
You'll need a strong belt/strap, scissors or a knife, lighter, and a scotch tape/thread and needle.
First, measure your head circumference with a tailor's meter or the strap which you are going to use and cut a bit longer piece than you actually measured, so you could tie two ends together. Tie it while on your head so it would fit it well.
Then cut two pieces off the strap whichever length you want for your training. These two pieces will hang down from the harness and hold your dumbbells or weight plates, so keep this in mind in consideration of their length. Tie them well to the sides of the loop you made for your head. I tied them right next to my ears so the straps wouldn't bother them when exercising.
Next cut a comparably short piece of the strap and tie it to the head loop behind the knots of the hanging straps. Adjust the length of it by putting the almost ready harness on your head and checking if the top piece is sitting well on your head. Don't make it loose or there is no sense in attaching this piece to the harness. You can even try the harness without it and see if it is OK. I put it there for additional support on the head.
Now use the scotch tape (sticky tape) to fixate all the knots of the harness in places. It's important to make them stable so they won't move during exercises. You can use a needle and a thread instead. For me it's much faster and simpler to use the sticky tape.
Now the final result is a harness which allows you to use weights in order to train your neck muscles.
I just tie a dumbbell to the hanging straps or even two dumbbells.
I use a hat to soften the pressure on my head from the harness.