Ask your grandchildren to design one. They will enjoy working out all the maths and measuring the garden and where they want to put it then tell them to ask their father to help with the heavy work and find some old crazy paving that someone's digging up and layer them on top of one another to make a small wall. That way they can all get involved and you don't have to pay a penny. Supply them with blackberry pies and orange juice throughout to keep them happy.