Just a thought...

I recently build 3 huge raised borders, each at 1.3x15 metres. I constructed the side out of any wood I could find, avoiding wood that had been treated. I mainly used palettes and old floorboards for the sides and insulated them from the soil using rolls dampcoarse plastic. Costs about £6 per roll of about 50 foot - the stuff is designed to keep out moisture...not totally green, but I will find many uses for it, should I change the design of my allotment in the future. Tough stuff!

All the best, James