My time has come. I mean--I was just doing a bunch of research on briefcases myself!
If you're really married to the idea of a dowel, there's a few I know of:
Dowel computer case #5297, approx 16x12.5x4.5 - note, this one pas a padded interior meant for a laptop... from the late 90s
Kensington briefcase #5279, approx 16x10x4.5 - I think this might best fit what you're looking for
and, the Beekman briefcase #5266, approx 16x12x4.5 - this one only has one large front pocket, but it would probably be the easiest to find
That being said, if you're willing to compromise on the dowel and/or two front pockets over one, there are a lot more options. Plus, no dowel means a lighter briefcase. I personally don't find the briefcases too heavy, but they
are larger, all-leather pieces.
I have a Prescott Brief myself and I love it, though it was a lucky find in my searches:
View attachment 4711584
A few no-dowel, single pocket options are the Metropolitan Brief #5180 or the Lexington Brief #5265 -- both are fairly easy to find in a variety of conditions. There's also the Letter Brief #5270, which is slightly slimmer, and might be good for someone who just wants to carry an ipad and a few other things and not a full-sized laptop:
View attachment 4711583 View attachment 4711582
(if you want some more pictures of the Prescott for an idea on size, since I own that one, I'm happy to provide

)