I used scaffold planks and being untreated should last for at least ten years. You can always tack thick black polythene to the inside.
Try and transfer soil form other part of the plot like thingybobby says. I ordered one and a half ton of top soil from a local guy that said it was screened and had no weeds, six weeks later they are full of weeds which i have now got under control. Its better to let them fill up over the next couple of years with compost and green manures.
