I need to reinforce a railing, above a stairwell, that is completely un-supported at one end (other than flimsy metal vertical stiles, which are nowhere near strong enough to keep the end of the wooden rail from being secure). Here is a picture (sorry it's not clearer) which shows the situation; it's the left-hand end of the rail that is the issue.
I plan to do it by running a 2-3/8" diameter aluminum tube from floor to ceiling, and screwing it into the end of the wooden rail.
To make this look decent, I therefore need to cut a concave indentation into the end of the wooden rail (so that the tube fits into it). The rail is roughly 2-2.5" wide also. How can I do this ?
Here is another photo, showing the situation better ...


