Works well with the model environment, with the 'loft command' (see attached .f3d)
the template for the plasma cutting will be a bit more of a problem:
- you could use the tool ExactFlat, its available online and free, and it allows you to unfold curved objects for laser cutting and such. But prepare to get your mailbox flooded with ads from ExactFlat
-For a duct transition pipe like this, you could also calculate the design: the transition pipe consists of four trapezoid shaped sheet metals, the long edge being the side length of your square duct, the short edge being a quarter of the circumference of your round edge, and the two tapered edges of the trapezoid will be the 'transition length'.
this will get you almost there.
for even better fitting, add some curvature to the short edge, resembling some devil horns, otherwise there will be slits at the end of the weld seams, where the round flange will be attached.
Hope this helps