Mads,
You can easily DIY a release weight with a brick, a screw, a large crimp, a swivel and a de-barbed large single hook. You will need to add a handline or some sort of line system to it.
A picture would make it seem really easy. All you need to do is attach the hook via the screw through its eye to a brick, the same way a release weight is molded onto a sinker. Then, crush the barb on the hook with a pair of pliers, and thread on the swivel first, followed by the crimp. Squeeze the crimp at the highest point on the hook so that the swivel can't get back out and there you have it.
Jamie, perhaps you can help me dig up a picture of this - I can't seem to find one anymore.
I have successfully released numerous 15-20kg+ fish with this, mostly seriolas during 'Sambo season' but also numerous large ungainly demersal fish like groupers and cod. There seems to be evidence that a release weight = lower mortality rate. A release weight is relatively simple to use. Additionally, unless the operator is trained to use a venting tool, the chances of piercing the guts, liver, or other internal organs in a fish are high, as these organs partially envelop the swim bladder. Furthermore, different fish have slightly different anatomies, so some require slightly different points and angles of penetration. I do not recommend venting via a needle unless highly experienced or under desperate conditions.
I have seen people try to do it only to have a bit of intestine poke out, and continue trying to blindly stab into the airbladder while turning the fish into a literal pincushion.
Best regards,
Kasey