John,
I've used a pair of 2" X 2" lumber (could use 2" X 4" as well) to lay across the lengthwise racks and act as cross bars. In fact, it was to transport two canoes so I needed extra width to get both on top, side by side.
Make sure you lash or tie them to the lengthwise rack securely so they cannot slide forward or backward any substantial amount. Then make sure you secure the canoe to the car as well.
Personally, I use the foam pads and multiple tie-downs and find they work great on every vehicle I've ever had to attach a canoe to. If need be, you get the foam pads with a groove in one direction on the top and a groove in the other direction on the bottom and you can slide a 2" X 2" through those and it behaves like a rack. Good straps and ropes in the appropriate locations work just fine with the foam blocks - I've driven thousands of miles with this setup for many years.