-- Last Updated: Jan-21-13 7:11 PM EST --
First off, I think my double-loop method might have an advantage on my rack that might not be possible with most, because I have secure anchor points for those side-to-side loops that are well out toward the rack ends. It simply isn't possible for the boat to move far, and it can't creep over time. I'd have to see your method to figure out why the boat can creep, but something is different from what I'm doing.
Another trick that I've mentioned before is to mount the canoe farther rearward on the rack than what "looks pleasing to the eye". If you place the boat so its centerpoint is about two to three feet behind center relative to the rack (the rack, not the vehicle itself), you'll reduce the effect of crosswinds and truck turbulence enormously (two feet rearward is usually plenty, but I might go with three feet in severe conditions). I can't recommend this strongly enough - it just works amazingly well. By the same token, if the boat is forward of center on the rack (usually, people do this to make the boat more centered in comparison to the overall length of the vehicle), that makes the effect of turbulence and crosswinds far worse. You've already pointed this out of course, but if I had to guess, I'd say moving the boat rearward of center by two feet cuts the effect of crosswinds down to about one-quarter of what it'd otherwise be (which is why I say it's "amazing").
Your idea for a two-piece gunwale block that locks in place via rope tension sounds very clever to me, and I bet there's a way to make it work, even if it means getting it locked with a separate rope (rather then the tie-downs). My gunwale blocks that fit 2x4s consist of a piece of angle iron that's a little longer than the width of the 2x4. They are padded on the surface which contacts the boat with plastic tubing having a short length of rope inside to keep it "fluffed" (this also insures that the little bolts on each end of the tubing which hold it to the metal won't contact the gunwales, though they are covered with thick plastic too). There's a downward-pointing bolt welded to each end of the angle iron, and those bolts go through holes in a matching, flat steel plate that fits against the bottom side of the 2x4. Tightening nuts on those bolts clamps the 2x4 between the top angle iron and the bottom plate. Since the bolts are welded to the top piece, assembling the bracket can be done pretty easily with one hand if necessary, such as while reaching to the back side of a canoe that's already on the rack, and tightening each nut is a one-handed operation too since the bolts won't spin. If I couldn't weld, I'd get bolts that were threaded their full length, and lock them in place with nuts on the bottom side of the angle iron.
Someday I'll replace my broken camera so I can illustrate things like this, rather than describe them.