-- Last Updated: Nov-30-12 2:26 PM EST --
So far, I only use gunwale blocks when using my two-boat-wide cross bars, which are made from 2x4s. Each block is a piece of 1" x 1" angle iron that's long enough to overlap the width of the 2x4. On the bottom side of the 2x4 is a flat steel plate of the same length as the piece of angle iron. Bolts are welded to the angle iron, pointed down, so that they line up with matching holes in the flat plate and clamp the whole device to the 2x4 (the bolts are welded for convenience only, but it's a really big convenience allowing one-handed installation in that hard-to-reach area between the two canoes). One nice feature is that the contact surface can be set at different angles to match the angle of the gunwale, so unlike factory-built blocks, there's a flat surface-to-surface contact instead of just one sharp edge of a flat surface in contact with the gunwale.
Steel isn't such a good contact surface for gunwales, so for padding, there's a piece of clear plastic tubing bolted to the vertical surface of the angle iron. That tubing has a piece of rope inside to keep it "cushy" instead of getting squashed flat, and the bolts heads holding the ends of the tubing to the metal are covered by short sections of split plastic tubing so they can't mar the gunwales (though the rope-padded section of tubing stays thick enough to keep the bolt heads away from the gunwales).
When carrying a single boat I use my standard round bars, and instead of gunwale blocks I make use of auxiliary lengthwise bars on each side of the rack as anchor points for loops of rope that serve to "pull" the hull both to the right and left, making the boat immovable between them. If I were to use gunwale blocks on round bars, I'd use something incorporating U-bolts. I'd probably use a rectangular block made from 1.5-inch square tubing (steel), having a pair of slotted (or maybe just over-sized) bolt holes to accept the ends of the U-bolt. Slotted or over-sized holes would make it possible to set the blocks at various angles matching the curve of the gunwales. I'd install gunwale padding the same way as described above.
With either type of block (the type I made for 2x4s or the type I would make for round bars), a person could make at least 50 of them for the same cost as a set of them from from Yakima or Thule, and making a set of 4 doesn't take very long if you have the tools.