Clamping curved bits of boats while the glue dries is always a bit of a nightmare. I have wasted many hours trying to persuade a pair of flat-faced and parallel jaws to hold things while they decide to slip off just when I think I've arranged them in a way that works.
My dad is building a ferry and hit the same problem. His solution was simple.
A big blob of Blu-tack on the jaws and then some worn out abrasive paper. This worked a treat, the paper providing enough grip and the Blu-tack helps sort out the angles. I'll remember this in future.