Welcome to the Bunnings Workshop community. Thank you so much for joining us and sharing your question about your outdoor bench seat.
Using Meranti timber is an ideal replacement for the existing parts of your outdoor bench seat. It is a naturally stable and durable medium-density hardwood. However, I still recommend using a protective coating such as Bondall 1L Gloss Monocel Gold Marine Clear Timber Varnish to protect it from the elements.