I am about to build a raised garden bed for tropical plants. The bed will be 8m long x 2m wide x .65m high. I’m making the front visible wall out of beset block. I want to make the sides and the back wall as cheaply and quickly as possible, I’m thinkjng of using star pickets and marine ply. Has anybody done this? If so would I need to worry about drainage or will it be fine?