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Welcome to the Bunnings Workshop community @Bea834. It's marvellous to have you join us, and many thanks for your question about filling a gap.
Given the size, I'd recommend pushing gap filler rod into the cavity. 5mm in from the front edge is about right. This prevents you from wasting excessive amounts of silastic within the cavity. Use some Sikaflex 11FC to fill this 5mm gap and smooth it to ensure a neat finish. Here's a helpful step-by-step guide: How to apply silicone sealant.
Please let me know if you have any questions.
Mitchell
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