I coudn't buy freediving neck-weights, so i bought 2x 2kg sandbags and attached them both to each other and around my neck
it's not good for streamlined DNF, but it helps me stay underwater on my full lung capacity a little
It’s super easy building a neck weight. I bicycle tube, some metal pellets, a couple of plastic clips, some cable ties and duct tape and you’re good to go!
This is in Swedish, but I think you’ll get it anyways from the pics. If you wonder about something, just let me know and I’ll do my best to translate.
https://juniordykarna.files.wordpress.com/2015/03/bygga-nackvikt.pdf