in-the-home boring jig

[bases-loaded@juno.com]

I am looking for any ideas that will facilitate boring single hammers in
the customer's home when the shank has broken off bluntly right below the
hammer head (vertical pianos).  I have trouble precisely reproducing some
of the angles, especially the compound angles.  I have been using the
inexpensive adjustable maple jig for years - sometimes successfully,
sometimes less so.

What have some of you found to be a successful approach to this problem? 
I have found that fairly substantial tonal difference can arise when the
angle of the hammer changes, even when the strike point is maintained.

Any and all suggestions welcomed!

Mark Potter
bases-loaded@juno.com