mindseye11 wrote:
Does anyone know if there is such a thing as rectangular flex ducting?
I am installing an over-the-range microwave and its vent slot does not
line up with the one currently in place through my cupboard. Since the
cupboard is wider than the microwave, I need to go about 6" over with
my rectangular duct through the cupboard. I figure the best way to do
this would be with flex ducting, or by cutting a straight duct at an
angle and fitting it. Anyone have any ideas?

This application requires 26 gage sheet metal because of fire reasons.
Any type of flex is likely to be plastic (not the best for containing a
fire). The corrigations of any flexible material will trap grease and
create a much nicer fire in your ductwork.
Take careful measurements and have a proper transition fitting made by a
local sheet metal shop. With their fancy new CAD/CAM plasma cutters
it's a snap to make anything you can think of.

Good Luck