I just bought a Berlin turbo XL and it was to tall to fit in my cabinet/sump. I put it behind the tank on a ledge and pvc piped it. It was a pain because the overflow needs to drop at a 90 degree angle for it to work correctly. The classic you have should come with hang on attachments. (The XL does not it is too large). Just call Red Sea and tell them you want to use your classic as a hang on and they should send you what you need.