How to install an extraction fan in a ceiling which has an existing hole larger than required

I have an existing issue with my bathroom ceiling fan, as detailed in this post: How to fix an ineffective Extraction Fan setup in bathroom

I now wish to change it to a Fan that:

a) Has a new motor, so it works b) Has an outlet vent that points upwards not sideways so the air can easily go to the roof vent

The IXL original (https://www.bunnings.com.au/ixl-tastic-original-3-in-1-bathroom-heat-fan-light_p4442438) seems to fit this requirement. However the cutout required is 262mm by 262mm as detailed on the third page here (https://2ecffd01e1ab3e9383f0-07db7b9624bbdf022e3b5395236d5cf8.ssl.cf4.rackcdn.com/Product/009c39d0-4f3b-4fd5-aacf-f58341f2d9d1.pdf)

My current ceiling aperture is 300mm by 300mm, which fits the Heller Silver Mason model (https://www.bunnings.com.au/heller-4-x-275w-led-silver-mason-3in1-bathroom-heater_p4441815)

I can either replace with exactly the same model, but I thought since I'm changing it over I may as well get one that has an outlet vent pointing upwards rather than sideways to suit the roof vent existing. Unless there is another reason they did not use one of these initially that I am missing (maybe debris that fall through the roof vent would get into the Fan?)

Any advice on how to fill the gaps that would be caused by using a smaller unit is appreciated.

Thanks