I have a lot to do, including bends, and I just need a 25mm mowing strip width, 4x1 seems a good way to go, but there is the warping issue. A non wood 4x1 or similar dimensions would be ideal. I want to avoid concreting, and wood or similar is more workable and quicker. The garden border is a trench, that will remain, the 4x1 will sit vertically to topsoil level