Front- and down-firing subwoofers -- Teufel Audio blog
Xcab Dual 10" Downfire Sealed Subwoofer Sub Box Enclosure For 1999-06 GMC/Chevy
Shallow Mount, Single Sealed – Atrend USA
Amazon.com: ALPHASONIK AS10DF 10 inch 1200 Watts 4-Ohm Down Fire Shallow Mount Flat Enclosed Sub woofer for Tight Spaces in Cars and Trucks, Slim Thin Loaded Subwoofer Air Tight Sealed Bass Enclosure :