Hey mate,
The plastic ones can be pretty dodgy and very expensive when fishing for kings-with damage and lost gear, some have lost Stellas.
We have rail mounted stainless steel ones and once they are tightened then i can hang off them and they won't move which is 80kg of weight so i reckon they are very good but the screw that connects them to the clamp comes a bit loose when your driving the boat in choppy conditions which means you just have to tighten them before you go back out again.
Not sure of the brand but they were ordered as extras by our dealer when everything was getting fitted on our boat. Might be an idea to talk to your dealer about it or just have a flick through some of the catalogues like BIAS and other boating shops, they have very similar ones which should do the job well.
Or you can get them welded onto the rails and the most certainly will not move.
Cheers
Josh