I have some foam blocks and they are OK for short trips but would not rely on them cross country.  I had to duct tape the pads to the canoe because the canoe teneded to slide back and forth within the pads.  And the lateral support was not great.

I do use one pair now with my pickup.  I have a T support that fits into my trailer hitch and I use the pads for the bow.  With the T I get the secure attachment point so the less concrete attachment of the pads is not a concern.