There are now ,,boards" made of recycled plastic. I have used them in the past in my former garden in Germany with excellent results to contain my compost pile. And I have seen the same material here in Portugal for sale, including fence posts. It is heavier than wood but can be worked the same way. And the material is strong and solid. It is of grayish brown muddy color so fits well in the garden. It is maintenance free and should last for a long time.