The crochet I made is definitely not shaped correctly. If you go into Google Images and search for "Roubo workbenches" you will see numerous examples of better ones with longer, shallower tapers that will clearly hold the workpiece more securely. Curiously, if you look at Roubo's plate here, for example, you'll see that his doesn't have a shallow taper.
Update: I have to admit that Dean, Bob and Adam are right. I made a "sideboard hook" that matches the look of the Nicholson bench to my eye. It's also somewhat wider than I usually see as I want to accommodate 8/4 material. As I've written, I enjoy experimentation. I'll likely make at least one more. Although the angle is only 45 degrees it seems to work fine and, in any case, is more angled than Roubo's. I continue to puzzle over why his is shaped the way it is. I can only surmise that he thinks this is enough of an angle to hold the piece against the bench, and maybe it is. Also shown in the picture below is my bench dog. :)