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Welcome to the Bunnings Workshop community @sandraboyd8. It's wonderful to have you join us, and many thanks for your question about shower bases.
You typically would use a 900 x 900 base, such as the Estilo 900mm x 900mm Shower Base 4 Lip Rear Outlet. That's just the max size that will fit on it. There is a flat raised section on the base that will accommodate a number of sizes slightly smaller than 900 x 900, such as your 880 x 900 screen.
You might be able to find a base of the exact size, but 900 x 900mm is far more common, so that's what most bases are designed for.
Please let me know if you have any questions.
Mitchell
