I have had success with using plain old MDF and then coating it with a two part expoxy called West System. It's used in the marine environment to seal wood. As soon as it is carved I spread on the West system on both sides and the edges. Let it dry , light scuffing, and then put a second coat on. You can paint over it and it's good for ever. I think I have posted some address signs for houses that I have done using this technique. Cheaper than any of the good weather resistant woods. Only downside is probably wouldn't work if you wanted to stain. I love using Lowe's metallic paints on these signs...comes out looking like it was cast in bronze.