Brian: Almost nobody uses waterproof panel board any more. PVC foam board is the way to go to make door panels. You can get it from most sign shops or from sheet plastic suppliers. It goes by the brand names CelTec, Komatex, Sintra, and Palight. It comes in 4' by 8' sheets and is available in 3MM and 6MM thicknesses. 3MM is about equal to 1/8" thick, just like the panel board. This stuff cuts with regular woodworking tools. When you drill holes, use spade bits or Forstner bits. You can sew through it with no problem. It is completely waterproof, unlike "waterproof" panel board. It costs about $30 for a sheet, and is well worth it.
No one lives forever, the trick is creating something that will.