Using a metal file, file up and down, following the outline.

Continue this, going around the outline until the desired opening is made.

This will most likely take quite a bit of time. If you have access to a small jab saw or similar power tool it will speed up the process.

Do this step ideally in a well-ventilated area as the wood shaving are small, and inhalation can cause harm or injury. Wear protective gear like a face mask, and goggles/eye protection to prevent injury or pain.