You may want to either coat the model with a uv resistant coating or make a casting of it in urethane with a uv protective additive such as sun devil from Smooth-on. Task 2 series they have is pretty durable and is easier to tint than some the other resins. There are other suppliers as well like Anchor Seal company, Alumilite and BJB enterprises. Find one that is closest to you so they can service you best.
I have used Anchor Seal and Been using Reynolds who is a smooth-on distributor because they are local.