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Welcome to the Bunnings Workshop community @ventodimare. It's fantastic to have you join us and many thanks for your question.
Any timber which doesn't have a chemical treatment, used for a wooden box which contains soil and moisture, is going to have a limited lifespan. Even chemically preserved timber like H3 treated pine is not designed to be in constant contact with soil and moisture. H4 treated pine is suitable for the purpose, but there are limited dimensions available and those sizes are generally only suitable for retaining walls and posts.
That's not to say you can't use Cedar just that you'll need to take preventatives steps to extend its lifespan. You'll need to ensure it gets a yearly coat of an exterior oil and I would advise lining the inside with a builders plastic. This plastic prevents the damp soil being in constant contact with the timber and can prevent extensive rot from occurring. Make sure you add drainage holes at the bottom for the excess water to flow out from.
I needed a simple way to hang a wooden planter box to a handrail and chose to screw a few Pinnacle Zinc Small J Hooks to the rear board. The allowed me to hook it over the handrail and move it around if needed. Please be advised you should only hang objects on the inside of a balcony handrail to avoid them falling and possibly injuring someone underneath.
If it is a timber handrail you could potentially screw straight through the backboard of the planter and into it. Let me mention the helpful members @Brad, @Adam_W to see if they would like to contribute some different attachment methods or comment on the timber choice.
Please let me know if you need further assistance or had any questions.
Mitchell
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