Filling it with masonry seems to be the best as far as compatible materials,
but before you do, think about the future. Will you ever have need to run
wires, vents, or anything else to the outside? If so, you may want to take
a more temporary approach.
On the outside, I'd use brick and mortar to match the existing brick as
close as possible. . Once sealed, you can even fill the rest of the void
with expanding foam spray. That would make it easier if you ever wanted to
add a vent for a high efficiency heater or power to an outbuilding. If you
want solid, build it up with some stone and cement mix, probably in a couple
of steps, unless there is a way of attaching a board to act as a form and
hold everything in place.
It's too big to camoflage with a dry vent cover, so see if your
hardware store has a cap you use when removing a chimney, a chimney
cap, an exhaust fan cover or something like that. Then insulate behind
it. That would be the cheap and easy way to do it -- not the best way,
just the easiest.
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