The new Bthroom robe hook we just received comes in two parts. The internal part came with three plastic anchors, each 2 1/2" long obviously designed to hang it on dry wall and secure the outer part to it. But there isn't room in our situation and we need the hook to go on the back side of a hollow core door. I'm not quite sure just how I want to do this. We're tall people and our Turkish bath robes have some weight to them. I've cut apart a hollow core door and seen diagrams of them and I'm a little "antsy" about what to substitute for the 2 1/2" plastic anchors. Any suggestions?