Funnels are complicated beasts. We tend to focus on the outstide--which is only natural.
Funnels often include the intake trunking to the boilers, which hleps preheat the air being pumped in.
There are various steam and similar exhausts run up the inside of the outside structural shape we see.
The actual "chimney" part of the boiler exhause is only a tit more than 25% of the stack volume.
Starting in the 20s, the naval fashion began to be that stacks were self-supporting, eliminating the need for guy wire rigging. In Naval use that structure became a place to mount more things, like platforms, masts, antennae and the like.
Once you assemble the stringers and purlins and the like to support the stack, the skin of the outside can be about any shape. When USS Fletcher was designed, USN ships were meant to be streamlined, aerodynamic, anythign to coax another knot of speed out and comply with the various Washington Naval Treaties.
So, Fletcher (and the Tamiya kit) have the poly elliptical plan form funnels. The "round" bridge reall isn't, it's actually elliptical, too. Per the naval architect for Fltech that shape was worth something like 1.5kts for top speed. And presented no delay in peacetime paced building schedules.
Bethlehem's layout team looked at the number of Fletchers needed and the expected delivery times and made a number of suggestion for streamlining construction time. Not all were accepted, but the scheme to use almost parallel sides on the funnels ant radii only fore and aft was accepted, on both East and West coasts.
This meant bending fewer plates and having to cut fewer curved stringers and frames, too. Also, the forward intake louver could be made flat, and not curved, and the after intake to a single curve radius, and not a double radius.
Bethlehem estimated this took two weeks off the building time for a Fletcher.