Indeed go with marine plywood or foam pvc, if you want to raise your bed higher you could do with that chipwood stuff, but you must cover it.
Yes i know i am really behind on my garden schedule.
Drawing example:
If your radius is 1 meter:
Drive a small stick somewhere in your lawn.
Take a piece of wire, make a nouse?(loop?) in it, so it can move free around the pole.
Measure 1 meter from the pole/stick to the other end of the rope, make a knot on that distance. Start measuring from the center of the pole/stick
This will be your centre track, for the edges of your wood, make two new knots, one 4 inch/10 cm above your first knot and one 4 inch/10 cm below your first knot.
Place youre sheet as far as your most outer knot can reach.
Take a pencil and draw a line for a perfect circle.
My English is not so good for this point.
I am busy in the garden anyway.
Here are some pictures to clear up some things:
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Hope this was of any help.
Best Igor