Thank you for the nice replies.
Tim, yes just going bigger, although a bit more difficult.
Art, the SignFoam is a brand name, It is high density urethane and I have been using the 15 pound density. (I buy mine at a local sign supplier, they also sell vinyl cutters, printers like Roland, ect.) and Yes it is water proof, it won't crack, mildew, shrink.
There are other companies that make this type of product, under other names. I will look for it and add it to this post when I find it.
For the paint, it is Urethane, primer, color (4oz. bottles) and clear coat from kustomshop.com and I buy mine thinned and ready to use in my air brush. The primer and clear are in Quarts size and a catalyst to mix with it. The paint kit is simular to the paints that your car is painted with. Polish, wax and hang the art outside.