Oil Cooler Ducting

High oil temps are leading me to change oil coolers (had the wrong one installed in the first place). I am installing an Aero-Classic 8001599 which has core dimensions of 6" x 4.33" and am looking for a pre-fabricated duct. Does anyone know where they can be purchased? Right now I have the Vans duct over the standard Niagara 7 row oil cooler and that will be too small. Thanks