You could use thin fiberboard, aluminum or sheetmetal. IŽd use the chance to put some tarmat in the doors and on the new panels to eliminate rattles and make better use of your speakers. You should put some fabric on the panel, use a carpet from a partscar or get some audio install fabric, that makes the doors much nicer when its freezing or when the sun is shining on the doors. You can either line the new panel up exactly like the old one and reuse the clips or you can use screws.
