Well the 5mm Chinese Birch plywood from Menard's did not hold up for one winter and spring. Obviously if I want wood grain I need to go solid wood or fiberglass and epoxy over wood.
I found a product that is sold to farmers and greenhouses and is available in large sheets. I have bought 3 sheets of HDPE (High Density PolyEthylene) to re-skin my sides. Sure, it's white, but it's nearly bullet proof.