Are you talking about the panel board itself, or what goes on the panel?
If you are talking about the panel itself, the best material I have found is PVC foamboard. It is also called closed cell PVC and expanded PVC. This goes by the brand names Komatex, Sintra, CelTec, and Palight. It is available in 4' by 8' sheets and also smaller more manageable sizes from sheet plastic suppliers, sign shops, and upholstery suppliers. It is inexpensive (about $30 for a 4' by 8' sheet), light weight (1/2 the weight of solid PVC), strong, heat formable, glueable, waterproof, and can be cut and drilled with standard woodworking tools. Large holes should be drilled with spade (paddle) bits or Forstner bits to prevent tearing. You can also sew through it very easily. The 3MM (about 1/8") thickness is plenty for door panels. It also comes in heavier thicknesses.
No one lives forever, the trick is creating something that will.