This is what I did which worked out very well. You will need to purchase a front hitch and then have a welder make up your goal post mount. Then you will also need one roof type rack. You end up with a very stable and strong carrier which will carry just about any length you want. As you can see mine also incorporates a bike rack.
This set up also creates a very easy loading system. The front goal post rotates so you load the front of the canoe on it first and then lift the back of the canoe and walk it around to the roof rack. That way you only lift one end at a time.