Follow up post, now that I have several trips on the new trailer.
The 21" bikeboot is perfect, holds the bike rock solid. The 17 and 19" are not the same design, and while they look like they would work better, I do not think they as good. they do the job OK, but I need to toss a single strap on the bike to ensure it stands straight up, otherwise the bike will sometimes tilt to one side. Not sure if it is how I put the bike into the clamp, but one tie down takes care.
My round trip to my riding site is 150 miles, mostly freeway and everything works fine. I can load and unload bikes as fast as I can move them on and off the trailer.
Overall a much better solution than tie downs, and I get compliments or questions on them every trip,
Mike