I have heard of going to hardware stores and looking in the HVAC section for heating ducts. There is a certain fitting that you fit a round hose to and it makes a 90 degree bend and opens into a rectangular opening for floor vents and such. They're popular for jimmy-rigged ram air scoops. If you get a 4 inch pipe on it it will probably not pose much restriction at low speeds but at speed the air being forced in should eliminate quite a lot of intake restriction.