rmd | What i did:

short term: tack up several layers of trash bag across the opening so
that cold air and pigeons don't fill my house. (no pigeons in sight,
but i don't really want to face one that things it's still a good
roosting place.)

next step: hide the air conditioner unit in a closet.
okay, i haven't done that part yet, but let's just remove the a/c
from the equation.

then, it's time for THIS OLD SCRAPHEAP, where you have to complete
a given home repair project using only the materials in the
house to avoid having to go out to home depot.

count staples in the staple gun: 18. look for more staples. nope!
none here, so i'm constrained to the 18. get several heavy duty trash
bags, several smaller white trash bags, string, a big bag of trash i
was going to put out for the trash guys next week (this contains a
bunch of fiberglass insulation that i pulled out of the ceiling of
the bedroom when i took down the ugly lower ceiling), a piece of
old wall panel that's about the height of the opening but a bit too
long, and - of course - duct tape.

score the piece of panelling so it'll be about long enough, then break
it on the score mark. cover all the edges in duct tape. wrap it in
two layers of white trash bag.

remove the temporary barrier. try to save as many of the staples as
possible, since we don't have too many in the gun.

take the twine, put a bowline at the end, and staple it to the bottom of
the box that surrounded the a/c so that it's well anchored. run it up
and down in a zig zag pattern a few times so that it provides a rough
barrier to large objects falling out of the box.

next, put the piece of panelling (wrapped in trash bags so it's mildly
weather-proofed for a few months) up against the string.
then take an empty heavy duty trash bag and put a bunch of fiberglass
insulation in it. remeber to put on gloves *before* starting that last
bit. then stuff the bag into the box so it mostly fills it. tear
pieces of insulation inside the bag and stuff them into the corners.
notice how you can no longer hear street noises from the opening and
decide it's mostly insulated.

take a bare piece of insulation and cover the opening with it. this
doesn't need to be in plastic since it's not going to be exposed to the
elements. cover this with the folded heavy trash bag i had earlier
used for the temporary barrier. staple in place.

run out of staples and cleverly RE-USE the staples i had carefully saved
when i pulled the temporary barrier off. (push them into the wood with
lineman's pliers and then use the face of the pliers to bang them in.)

no big drafts, no noise, and the area is no longer a pigeon haunt.

now i should look and see if i can find one of the framed prints i
have that's big enough to cover the hole.

long term: when the weather's warmer, hire someone else to do the
necessary high-up-work to repair or replace the box and re-install
the air conditioner.