Ventilated vinyl soffit is available at Home Depot, Lowes, or a roofing materials supplier that also sells vinyl siding. It comes in 12' strips, similar to vinyl siding pieces. You simply cut each piece into thirds and you will have 48 inch pieces. Interlock as many as necessary to acheive whatever length of soffit you require. J-channel installed on the fascia and on the house is all that's necessary to slide these into place and create the type of soffit you are looking for.