We used Kopal clamps on a Hmc at work to hold 350x350x25mm plates for machining and they gave great clamping force but I would recommend making some steel bushes that you can knock on the knurled part of the screw to increase the strength of the Allen socket because they are prone to breakage if you over torque the screw so the coller stopes it ripping apart and becoming useless. They will be a pain to get off if you want to service the clamp but it's doable.