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Building a raised vege garden

Wanting to re-enforce an existing garden and build it up with the right boxing/framework to support it. Garden Length is 6M Width is 1.7M Depth is about .6M (60cm) side of fence and about 40cm near side to house. There is a Gap of about 10cm between fence and Garden edge. (so garden is not up hard against fence panels) I would like suitable thick treated (?) timber to support this garden all around it. Ideally would like to raise it higher.