Double cut foam blocks suffice to carry a single hull on a vehicle with factory roof racks. The blocks are positioned on the cross bars, and the rails slipped down into cross cuts. Strap the hull down as above, using double bow lines.
A canoe may be strapped to vehicles without roof racks using the same foam blocks. Two straps run through the car doors or rear windows (open the doors before running straps through). Position the blocks directly under the straps and use double bow and double stern.