Large 5' x 12' sign - material?

I am quoting a large (5' x 12') sign for a town in my area. This will be in a high profile area, so it needs to look good and be durable. The design is simple, solid black background with white letters and white logo.

I want to make the background as seamless as possible. The only material that I can find as a single piece that size is a 1/16" thick piece of aluminum. I would have to paint that and paint adhesion could be tricky. My other 3 options where I would have to join panels are 1) Extira, 2) HDU 3) Alumalite.

The Alumalite is offered in factory painted black. It has a corrugated core and can be up to 1/2" thick. I talked to their rep and he said that you can join panels by inserting dowels for alignment. I can then reinforce the seams on the back side. You will have a seam, but the sign is 40 feet from the road and at about a 20 foot elevation. The rep is sending me 2 sample pieces with dowels for me to play with.

Does anyone have any other ideas for such a large background? The good thing is that I already have a 2"x2" welded steel frame structure in place that was used for the old sign. That frame was welded to 4"x4" steel posts embedded in concrete, it is solid.