I’ve occasionally had a need to put clamping pressure on a spot that was out of reach of any of my clamps. Since I could never justify the cost of long reach clamps, I add an extended set of jaws to my C-clamps.
Start by cutting two pieces of hardwood that will reach the spot you need to clamp. Then drill a hole through one end and cut a notch to fit around the clamp body. You can drill holes through the clamp body at points that will provide the throat distance you need, accounting for some deflection of the arms. I glued pads to the bars and then attached the bars to the clamp body with a nail. When you’re done, you can remove the extended jaws and return the C-clamp to its intended function.