After going round & round on this, I think I am going to stick with my current K40 for a while longer and see what I can do to potentially improve things with it (and that plastic vacuum cap on the tip is a great idea). Beyond that, I have come to the conclusion that the tennis ball approach will likely not work for my current set up. As you can see in this first side view shot, my K40 is already mounted with quite a bit of rearward rake:
And in this second shot, I am pushing forward on the whip about as much as I can without putting a bend in it, and it still isn't coming close enough to the upper rear edge of my top, such that a tennis ball in that position wouldn't even contact the top to initially help absorb some of the whipping motion.
So, I may mount the K40 base down lower so a tennis ball or stand-off bracket may be able to help. If that doesn't work, I may end up going with the shorter 2 to 3 ft K40 fiberglass antenna in the original mounting position on my Hanson tire carrier.
Thanks again,
Don