This is a common method to protect desks such as yours, it spreads out the clamping force and reduces the risk of the clamp punching through the desk surface. There are even products available if you'd like to have it look more professional or just like the ideal of using sheet metal. That being said, if I were in your shoes, and because I'm of a frugal, diy persuasion, I would sandwich the desk with the clamp with two pieces of ~12mm plywood extending ~5-7cm on each side.