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Welcome to the Bunnings Workshop community @Gordie. It's brilliant to have you join us, and many thanks for your question about Merbau planter boxes.
There have been dozens of wonderful projects shared on the community showcasing Merbau planter boxes. If it were a particular project you'd seen and could link me to it, I'd be happy to work out the materials used. I suspect that if someone were to line a planter box, one of the most convenient items would be Builder's plastic. Just remember to pop some drainage holes in the bottom of it so you don't end up with a pond.
For a planter box, the boards would be spaced quite close together, even touching.
Please let me know if you have any questions.
Mitchell
