Try shelving boards, the ones glued up to create larger widths.

Plywood would be a last resort.
It does not miter well and you should try not to leave and edges exposed.

Even pressure treated plywood with waterproof glue will wick water into the edges and tend to blow off the paint.

The people who build the jaw droppingly out of place, ugly, Mac Monster mansions here, with the two story high doors and the giant columns use premade columns made of foam.

Azek

My research tells me MDO (medium density overlay) would be the best material. Its the stuff road signs are made out of.