You can never have enough clamps, especially when building a boat.
Here's Platt Monfort's method of making your own from plastic pipe.
Step 1: Get a Big Pipe and Cut a Bunch of Rings
Any plastic pipe will work. This is black ABS. Polypro would be nice. Never use PVC for anything. Get the size and thickness of pipe to match your clamping needs. Cut the rings wide enough to give you the clamping force you need and so you can spread the clamp with your hands.