I've found the best way to keep the cart in place is to have a separate strap (or rope) on each corner, going to the yoke and the thwart.
If the strap merely goes over the hull in a loop, even double stranded, it doesn't prevent the cart from sliding. It has a similar profile to a watch, and the canoe is the wrist.
I like to set the cart about two feet behind the portage yoke, then use a doubled strap to loosely secure each front corner to the portage yoke. (One strap doubled on each corner to the yoke.). Then I use a separate rope to secure each rear corner of the cart to a thwart, carry handle, or seat hanger. Finally I go back to the front straps and tighten them. But not tight enough to break something.