If the crossbars on the rack extend wider than the canoe you don't need anything to secure from the sides. In other words you can tie a fixed loop on one end of a rope long enough to double over the canoe, slip the end of the rope under the cross-bar and over the canoe, under the far crossbar and finally thru the loop on the end of the rope, tighten, and tie a cartoppers knot. This will secure side-side.
Then do the same with the front and back and you're good to go. The idea is to secure both laterally and linearly during movement.
Generally a compress-able material between the boat and the rack will make for a more secure footing. The foam pads are a good way to go. Some guys put pipe insulation onto the rack crossbars instead.
I think the most comfortable PFD is the inflatable sospenders, you might get more opinions on that though.