Ray, I've been trying to figure out how to squeeze a refugium into the lil area I have I considered a HOB refugium, until I picked up my 5 gallon RO container and WHOAH waaaay to heavy to hang on and anything under 5 or 10 just seems to small to be helpful.
Hehe my only point in all this babbling is that if you're going to do a HOB refugium it's gonna be really small, unless you're going to have it hang so the refugium actually hangs inside the tank...... which is probably what you were considering already...... alright don't mind me =)