[brucepick]

OK, I've posted this here before, but since you asked:
Thin plywood.
My current preference is lauan plywood in 2.7 mm or 5.4 mm. About $7.xx per sheet in 2.7 mm., and $15/sheet in 5.4 mm at Home Depot etc. Very lightweight in those thicknesses.

I've had no problems with even bare wood deteriorating in weather. However finish paint will likely peel off so I now prime it with house paint oil base primer, this makes it highly weather resistant. If you plan to topcoat it a medium or dark color, have them pigment the white primer to as dark a gray as they will go.

I've used Duplicolor paints on top of the primer. On my grill panel the finish paint didn't stand up very well; it needs some kind of protective finish.

Duplicolor pickup bed liner black is my favorite. Very tough, dries in a textured finish. Bonds well to the oil base primer. Of course it bonds well to most other finishes also. Dries very fast, maybe 10 seconds?? so you can do a quick touchup or whatever very quickly.