Bob Engelhardt wrote:
... here is what I would do ...
Fit the mount to the tube and fill in the notches with weld.

I was so taken with this idea that I had to try it. View from the
outside (front):
http://home.comcast.net/~bobengelhardt/weldfront.jpg
and from the inside (back):
http://home.comcast.net/~bobengelhardt/weldback.jpg

The shaft is 1" diam, the notch was drilled with a 5/16" bit. A 3/8"
notch would have made it easier to get weld deposited in the back/inside.

I know, the weld is cold. But that was the welder's fault, not the
technique's.

Bob