Hi,
I have two  sheet metal nibblers, both cut a maximum of 18 gauge steel. 
The simpler  one is what I'd call a standard hand nibbler, looking like a
pair of plyers with the cutting blade at the top of one arm. This needs
about a 3/8ths hole to start, and cuts maybe 1/8Th inch by 1/4Th inch chunk
eachtime you close the handles. It kind of "stands up" on the work and you
squeeze the two arms together to make a nibble.
The closest modern one we see is from Digikey: G-C electronics 12-1806-000
This is O.K. for smaller projects but produces a very ragged sharp result
and wears out the hand quickly.
I believe ducting is thinner than 18 gauge, so it shouldn't have any trouble
there.

the larger unit is from Harbor Freight and sorry we don't find a modern
equivelent. It looks much heavier and instead of standing up on one leg from
the work, lies down kind of like a pair of tin snips about a foot long. This
one nibbles maybe 3/8Ths long by 1/8Th wide cuts. This comes out in  a
continuous strip as i recall, although the resulting surfaces are probably
slmoother than the smaller unit, they're still very sharp.

Since ducting sheet is, I believe pretty thin, I don't think you'll need a
monsterous tool unless you're doing a bunch of holes.

If you need round holes of less than maybe 2-1/2 inches diameter, you might
find a hole saw in your drill a much faster easier and safer tool for the
job.

Another option, and again, I've no experience, are these modern "multi-tools
that oscillate the blade instead of spinning it. We've just ordered a
cheapie one by Chicago from Harbor Freight that has a semicircular metal
cuttting blade as an option.
At least with an oscillating tool, you might be able to follow it and not
get shreaded. Not sure how a bat would track a power nibbler safely.


If I were going to do much nibbling in thin sheet, I'd sure wear gloves and
have some way of being sure i removed all the cutouts from the work area,
they're nasty sharp little devils.

Sorry I can't come up with more specific reccommendations, but therese a
start.
You might get more responses from the blind-handyman list
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Tom Fowle  wWA6IVG