A, I think they're brilliant, especially for dynamic, 'cause you can place them on the back of your neck, so they don't flap around under your chin, while you're looking downwards during your dynamic glide.
How to find out how much lead you'd need for what type of diving (pool or depth), for me it was just trial and error. If your body has stored a lot of fat, it weighs you up. If it has a lot of muscle mass, it weighs you down. It needs to be enough weight to assist you in staying level at circa four feet depth for dynamic, but not so much that you hammer down into the bottom of the pool during your glide. For depth, something like neutral buoyancy around half your target depth is a classic advice.
B, to build one, you don't necessarily need a mold. I bought four small 0.5 kilogram leads from a dive shop which were rectangular (the type for pocket Scuba BCD's), and I basically just gently hammered them into a curvy shape that fit my neck (lead is somewhat soft, or though they can crack if you're not careful). I had them locked in a vice clamp and just used a big metal hammer.
Try to find a number of weights that can be placed evenly across half the circumference of your neck (I was lucky with four, a two-plus-two setup), balance is everything.
You then put some strong nylon band the same width of the weights (from e.g. a sewing appliances store) around that in a length, that matches the circumference of your neck, and then some, so it doesn't sit too tight on your neck (with or without a suit). Put a couple of large click locks same width as the weights (also from sewing appliances) at the intended ends, with the ends of the nylon tape inside with the weights (not at the locks, make a double layer of the nylon). You can tape it all together to facilitate the next step (just know, that no tape holds in water).
Then you cram a motorcycle rubber tube with a proper diameter out over nylon and weights and locks, to hold it all in place, and cut the tube to length (a couple of inches out from the weights either side).
A couple of strong plastic strips at the bases of the rubber tire, so it doesn't come apart, and you got yourself a horseshoe.
See the image, if that didn't make sense. (And may I offer thanks to world champion Jakob Hansen for assistance in building my own, to succomb to name dropping.)
