Your sign guy's letter are flexible but not pliable enough to stay on the
inflatable surface.

The marine stores sell special letter that will stay on much better. I've
seen them in West Marine catalog.

Myself, I used stencil and a waterproof bold felt pen to put mine on.. They
fade and I have to redo them about every other year since my dingy stays in
the water/sun all season.

The plastic sign seems to work for a lot of people..

Steve
s/v Good Intentions