In reply to JimR:
You can pick up ratchet straps cheaply from any hardware store.
Depending on weather, distance travelled and travel speed, just strapping them securely to the roof bars (or failing that, just through the rear windows as long as any rear passengers aren't going to want to get out in a hurry) will be fine.
Make sure the straps are very secure and that there aren't any loose ends flapping around. Soft straps are not going to scratch a car. If in any doubt or if traveling long distances, factor in stopping every so often to check the straps are still secure.