I haul up to 3 canoes at a time on mine. THULE (two lee) had a kit that attaches to the existing rails. It is made up of 4 attachment points, 4 "feet" and two bars. You simply us the rails already on our cars and install the kit. they pop on and off very easily.
I have used this and many other THULE set ups over the years with my different cars and had few to NO issues. I typically do not tie the bow and stern, but will if I am traveling on an expressway for any distance I tie off the stern to the rear hitch. The front of the canoe comes no where near the front bumper on the roadmaster so there is real no way to tie it off unless you added an attachment point.
2 canoes on the car, they are both about 16' long. The two large canoes Green & White are 17' and 17' 6" and both have been on the wagon together.
My first wagon/THULE set up. I often stack 3. This gets a little tricky.