Since you're already makeing eye contact isn't a simple "thank you" even easier? What's there to feel awkward about? ..... a nod of the head, a smile, and a thank you... I think this would be much more appreciated then some disconnected sign. If you're able to speak, a nice warm heart felt verbal thank you would mean the world to these guys and gals.
But again ... JMO.
To those who can not speak .... I'm sorry, and the sign is by no means a disconnected thank you.