I need a simple fix to cover the ugly gap left after the oven was replaced. 600mm x 40mm approximate
Welcome to the Bunnings Workshop community. Thank you so much for joining us and sharing your question about what to use to cover the gap under the oven.
Your new oven looks fantastic. To cover the gap, I suggest using a Metal Mate 50 x 3mm 1m Aluminium Flat Bar. Place the bar at the bottom of the oven where the gap is, mark with a pencil where you want to cut the bar. After cutting the Flat Bar, sand the rough edge of the Flat Bar until it's smooth and safe to touch.
I recommend using Selleys 260g All Clear Multipurpose Sealant to attach the Flat Bar to the gap. Place some Bear 24mm x 50m Interior Decorator Masking Tape on the Flat Bar overnight. Once the silicone has cured you may remove the Masking Tape. Please keep us updated, if you have any other projects you would like to share please don't hesitate to post.
If you need more advice or information, please let us know.
The idea is to cut it ~6mm wider all round, then bend in the edges say 3mm* to form what will look like a rectangular vent panel that you will press into that gap. (* adjust size to suit the amount of protusion for your situation.) If you make it sit flush with the cabinet you would only need to bend in top and bottom edges to grip the edges of the hole that was left, but it would look neater if you cut the wire panel a bit wider, as I suggested and bend the sides right back around to form a tidy edge, then bend the top and bottom in 90° (a bit less, so it will spring out against the hole edges when you push it in.
Added: I just noticed that the mesh also comes with finer 6.5mm holes: