I make raised gardens out of discarded pallets. There is a business not far from us that puts them outside their premises for people to have free. I dismember them and re-cut the timber to suit. The result is a raised garden about 800mm by 500mm by 300mm high. It takes about three pallets to make each one. The timber is not treated so it will not last forever, but the price is right (!) and there is no shortage of them