If you want to read a number of posts on this same topic, search for a thread I started on it about six months ago.
Upshot is that only one company makes the 9" x 9" vent to match the size of the original Hehr Boler
one, and it is (unfortunately) pretty cheesy. It's made by Elixir.
So many folks, like Chester, have enlarged the hole and gone 14" x 14"
Coming from the boating world, I have to say that even the 14" x 14" ones I've seen are a tad on the "un-sturdy" side, but at least they are better than the aforementioned 9 x 9, and you do have a number of choices and also replacement parts are readily available.
If you are looking for a low, flat, metal topped one, Vintage Trailer Supply has one of the big vent companies making one for them with a flat, metal vintage-looking top (in the 14 x 14 size).
One the less heavy marine hatches would be very sturdy, but probably overkill and more expensive as well.