You will need to build a manifold like you say. I don't know of anything available off the shelf.
Even a 120mm pc fan doesn't have a lot of power to push air through much duct length. I hope your duct is short, if not, try a fan on each end in a push pull config.
If you go walk through the heating section in a home depot or whatever you might find a duct adapter or something you can easily modify.