Thanks for all the tips. I wonder why these are so difficult to find?

Guess I'll have to make one. The bolt idea is quick and easy, but that
doesn't leave me with a 1/2" shank, unless I am missing something.

I considered just taking a 1/2" peice of round stock and machining one
from scratch, and then buying a die for the threads. Though I may go
the route of buying a MT2 to 3/8-24 arbor and turning down the MT to
1/2". That should be a piece of cake as long as I can hold the piece
in my chuck well enough to turn down the whole taper. Maybe I can
mount it into my drill chuck first, then hold the drill chuck in my
lathe chuck. Hmmm.. I'll figure it out.

Thanks again.