I copied your idea last summer (thank you :thup), and tested it out on Powell. Worked wonderfully. I started out with Schedule 80 PVC tubing (1" IIRC), but then decided to change to wood as I just prefer handling it. Went with an ash, replacement tool handle. It's heavy, but I like it. The PVC would also have worked well, I believe.
A couple of details I used:
1) I kept the "stalk" long (tall) enough so that the fender whip does not touch the boat's gelcoat, thus no ability to mar it and/or no protection needed.
2) I made a hole straight through the stalk, so that I can pull the fender whip through and knot it (figure 8), thus making it relatively quickly adjustable (by changing the length of the whip).
And to think I considered eliminating the rod holders when I was doing my deck work (I don't fish, but they came on the boat). Turns out I use one for a fender "stalk" and the other for my ensign so they are very handy.
Here is a so-so photo (I'm not at the boat to take a better one). If you ignore the white lines and just look at the darkish one, that's the fender whip.
