PS,which named polystyrene,was the best alternative to wood 

High quality hot tub side panels and spa control panel Features;
1.Good property:waterproof,damp-proofing,moth-proofing,mildew-proofing,UV-proof,fire-resistant
2.Good surface feeling(the same like wood),can be sawed,planed.glued,nailed,recycled
3.Customized shape:easy-installation,maintenance-free,long-life to use(up to 20 years)
4.Green,Eco-friendly,Environmental products
5.Upto 10 years and maintance free

The advantage of High quality hot tub side panels and spa control panel

1.100% Made in the innovative panel design makes this kit easy to install

2.Each kit contains 8 side panels (accounts for 2 per hot tub side), 4 corners, trim pieces and 72 stainless steel screws

3.The flex corner kit will fit any corner radius from 5" to 12"

4.The exact same material that we sell to the world's leading hot tub manufacturers

5.Many  options to choose from: Redwood, Coastal Grey ,Weathered Acorn and so on .