Hi DM&FS,
> foam blocks are pretty worthless.  The cheap rope that comes with those kits
> doesn't hold a knot very well and unless you use  cam straps for the bow and
> stern tie downs, yu're really risking it.  Imagine a 70 pound canoe blowing

True: some ropes are better than others.
Also true: installing a rack does not equal proper tie down.  The
devil is in the details.

Don't buy into the expression: if you can't tie knots, tie lots.
Learn to tie knots and keep it simple.  Tie the lines with
independent, secure knots so that if one line loosens, the others stay
tight.

Tie the bow and the stern, as well as thwarts or seats to prevent
lateral movements.  Ropes over the canoe are essential because you
don't want to trust that seats and thwarts will not be torn out of the
canoe.  Ensure you have at least one forward facing tie down and one
rear facing tie down.  (Fore and aft springs.)

Personally, I use cam straps, good quality climbing lines (4 to 6mm),
as well as 1" webbing.

g