I have used a round duct on my machine and it turned out good.
pictures
here
I used a In-Win Q-500 case for the base.
also where do you plan to place your heater core?
i was looking at using the cube as my next case but decided to go with
Addtronics W8600. It gave me a lot of play area and I like the style.
when i was considering using the cube as a case i was going to move the powersupply to the top, put the heater core below it to exaust out the back.
then build a duct from the side for an intake to go to the heater core.
my $0.02 worth