I would aim the light more at the plant:)
seriously I would just put a plastic cup, or container, (punch it full of hole) in the refugium, fill it full of lava rock and let the roots dangle in the rock, but in reality as long as the pothos does not fall in (that will kill it) just having the roots hang in the water is enough. You might want to put some sort of climbing "fence" behind the plant to have it grow up something as opposed to all over the place. Instead of that I suppose you could just trim it often.