At lunchtime today I knocked up a quick clamp based on the cardboard template I made the other day and after work this afternoon I went down to the boat for a cup of tea, a snooze and to see how the clamp fitted.
Here is the clamp almost completed.
The metal plate is held in place by a nail and can move as shown.
It fits over the rail well enough...
... but there is quite a gap between the lower arm of the clamp and the coachroof which will need a spacer.
Like this. With the spacer in place the clamp, when tightened up, is quite secure. Without the space the clamp doesn't work very well. I'll do that when I get back to the workshop and tidy up the clamp itself now that I know it will work.
So, a successful trip to the boat for all three tasks.