to the forum!
My last tt was like your's and A&E was furnishing the arm with the knob for free. I had to go through my dealer to get it. It took my SIL and I about 10 minutes to put it on.
That was in a couple of years ago. I don't know if A&E will still do it, but it's worth checking into.
Oh yeah, there are two different sizes. Measure the bracket mounted on the side of your tt. That will tell them which one you need.
Hope this helps.
P.S. Read this thread on the same subject. Good info that should help you.