Looking to order a caporra ceiling hood, the exhaust is rectangle from the motor.

I am presuming it's easier to core through the house with a circle it'll be neater etc etc, even with builders employed than trying to form a rectangle shape in brick/brick.

 

With that in mind when should the shape change to circle? From the motor, before the exhaust? Not at all and tell the builders to get on with it...

There's plenty or rectangular > round adaptors for domestic ventilation, so I'd be more focussed on what external grilles tickle your fancy and then work the problem back from there.

 

If your builders can't stitch-drill a near perfect rectangle then you've got the wrong guys afaic.