I can remember seeing a unit derived from old Altlas Lathe company
plans, it amounted to a double-ended arbor and pivot to which you
bolted a change gear on one end of indexing and your gear blank on the
other. It had a plunger arrangement to index on the change gear. It
was gripped in the vise of the milling attachment. I think it was in
an old HSM magazine. Super-cheap and fast to make up, downside is that
you can only make gears of the same tooth number that you already have.
You can always use the old bandsaw blade and wooden wheel method, make
the circumference of the wheel just big enough to hold a blade of
however many teeth you need index positions and use the gullets to
index on. Angular error is reduced the larger you make your index
wheel. This is in one of Guy Lautard's Bedside Readers.

Another method would be to use one of those 5C collet indexers that are
cheap and make up a stub arbor. I've used flycutters, they're slow but
reliable. Keep the speeds down for best tool life.

Stan