Hi Lambis,
I have a 4lb (2kilo) neck weight that I use. I use a webbing strap and buckle system with little 1/2 lb weights. I've seen people use bicycle tire tubing filled with lead shot as well.
I'll try to post a picture soon. No digital cam yet, so I've got to develop and scan pics...
As far as weight goes, it depends on the diver. For me it takes 4 lb to make me close to neutral boyancy after packing, however one of the other freedivers in our club here in Toronto only uses less than 2lb. He's bigger than me! You'll have to experiment. What I did was just use different weights on my belt in the pool to find neutral boyancy with packing, then had my local dive shop put this neck weight together for me using the amount of weight that I found worked for me.
Aaron