You could tie the front as well, but consider
what I suggested:
With the triangle - you are pulling the boat into the
truck bed, so by putting it at an angle, you pretty
much assure that you won't have any sliding. You
are putting pressure from the back to wherever
the nose of the yak is.
If in the middle, you pull it tight enough - but not
so much that you see tremendous strain on the
handle, you can see what it's like by driving around
the block. Do a couple swerves to see if there's
any sliding of the yak.