Clamping device, friction between hard rubber and dirty steel

Hello guys!

Below short description of the device (according to attached sketch), then questions:
1. Fixture is responsible for clamping and transporting already clamped steel box after welding process.
2. Fixture works like a scissors. Cylinder create clamping force C coming from force S.
3. Fixture clamp welded box with clamps made of hard rubber.
4. Fixture can carry welded box thanks to friction force T created on contact surface between steel and hard rubber.
5. Friction force has to counteract for the weight of welded box. Equation for friction force according to sketch T=C*f (f-friction coeff.)

Question:
1. How can I include area of clamps in equation for force T. In equation I know I include only force and friction coeff..