What to use??

I have to make a 24 x 20 carved sign that will be hung outside in Michigan's Upper. I was going to use PTL 2 x 6 or 8's glued up with Titebond Ultimate, but I can't find any that is good enough to use for something like this. It has to be 1 1/2 thick at least. Do I dare use my usual pine panels and spar varnish it with about 4 coats? Or ........... any other ideas??