The simplest type of raised bed is as Bob Flowerdew of the amazing name has: mounds without edging, and he grows some crops on the slopes and others on the top. I think he refreshes the edges each year. Even simpler is to grow on the flat.
We have recently made some beds in our polytunnel and we used scaffolding boards. You might be able to get them second hand or scrap, or you can buy them from Wickes. They are nice and sturdy, about 9 inches deep, one high for a low bed, stacked two high for a deep bed.