I am sorry I have had to remove the image AQA complained that it was breaking copyright. It's the one of a number of students sitting under a tree.

Very eco-friendly isn't it? All you would need to do here is drive four small logs or square posts into the ground. Nail (better still wooden dowels) pieces of wood along the sides of the logs and nail (or dowel) a top on. Do just the same for the seats. Sorted. Look below to see it. (I have used square posts because it is easier to draw. You could do the same with plastic. The only difference you would have to fix the stuff together using self tapping screws. Galvanising is coating steel with Zinc preventing rusting. Wood preserver could be used to stop rotting.

From This

To This

Easy Done (see below)

  Wooden dowels

Galvanised nails

       Wood preserver