Welcome to the Bunnings Workshop community. Thank you so much for joining us and sharing your question about what screws to use to mount your power board.
I recommend bringing your powerboard with you to make sure that the screw head fits into the back slot. I suggest using the Pinnacle 3.5 x 20mm Black Round Head Timber Screw - 30 Pack. This particular screw comes in longer sizes if you need to go deeper into the wood. If you have any other projects you are working on please don't hesitate to post. I'm sure our ever-helpful members would be more than happy to provide advice and information should you need it.
If you need further assistance, please let us know.