HI Jim,
Here is the part you need: http://store.catsailor.com/tek9.asp?pg=products&specific=jmnpkpmrc8
You cannot mount it under the tiller bar as it won't swing from side to side.
On top would also cause problems as you have pointed out. So, put it on top but angled slightly forward, enough that allows the tiller to lay on the deck without stressing the Yoke Assembly.
In doing so, howeever, you will not be able to drop the tiller off the back of the boat (which is good) as that would cause the same stress you are concerned about.
For the actual tiller, just go to a local hardware store and get an aluminum tube the size you want.
Good luck,
Rick