[Butcher]

For thickness it's normally a tradeoff between weight and thickness, particularly with copper, the best coldplate material (unless you can afford silver). In theory the thicker the better but diminishing returns starts to kick in badly above 8-10 mm.

As for hotplates, you don;t need them because your waterblock is bigger than the peltier (right?), whereas you need a coldplate because your cpu (or gfx etc.) slug is small.

As for clamping force, yes they do work without, but if you check OnDaEdg's article on the main page he got a significant improvement from using proper clamping force.