Heh, heh! I wondered if anyone would catch that. Actually, with a bit of
oddleg fixturing, I can invert the platen during machining... it's just
got a bunch of 'interference' all over the top.

This mould has material applied from the top, then pressed out by ejectors
through the bottom. That's the 'why' of the inverse taper.

OK--I agree with the D reamer concept. I've used them myself for oddball
tapers. Also
made a few fluted reamers over the years.

Here is one more possible method, assuming you have DROs on your mill:
Purchase a tapered endmill in the appropriate taper. Get one with a little
size/stiffness to it. I use a 3/4" x 3" long 2 degree for a lot of draft
work on thermoforming moulds.

Rough in you holes to the minor diameter. Figure your "toolpath" as a bolt
circle, the more
points the better. Circle diameter is minor diameter minus tip diameter of
cutter. Machine
then hand blend as needed. If you use a large cutter, very little blending
is required.