Preheat oven to 150°C (fan-forced). Spray an 18 x 28cm slice tin (base measurement) with cooking oil spray.
Line base and sides with baking paper, allowing paper to hang over sides.
Put rice puffs, cranberries and sultanas in a large bowl. Stir well. Put egg whites in a medium bowl and whisk with a fork until frothy. Add milk and whisk until smooth.
Add egg white mixture to Rice puff mixture and mix until well combined. Pour into prepared pan and smooth surface.
Bake for 30 minutes or until set and golden. Set aside in pan for 5 minutes, then use a small sharp knife to loosen sides.
Leaving on baking paper, transfer to a wire rack to cool completely. Cut into six pieces to serve.