I was just looking at rail mount clamps on Amazon and found hundreds of varieties, some for motorcycle handlebars and other uses which would work just fine on a boat. Seadog makes a nice one for boats that could allow the panel to be adjustable on one axis.
All would come with the caveat that using them on a handrail might be bad. The hand rail serves a purpose. The moment may come when you will wish you had an unobstructed hand rail rather than a solar panel.
Mark