I wouldn't do that unless the cardboard was very strong. The round cardboard ducts I use in some speakers are strong enough you could just about use them for a baseball bat. Partly this is because they are thick but it is also because they are round. That shape provides some strength. Flat sheets will be much less rigid, and easier to vibrate.It is real easy to cut a few pieces of scrap wood to form your duct. I guess I'm just not understanding why you would want to use cardboard instead.