Replacing a Roof Vent

I have a powered vent fan on my roof that is dead. I am wanting to replace it with a Ventilation Maximum 301-12 passive vent. The hole in the roof once the powered fan is removed is about 16.5" and round-ish The passive vent only needs a 12"x12" square hole and has 19.5" diameter flashing so I'm only covering the hole with an extra 3" (1.5" on either side) which I'm pretty sure is not safe. Is it possible to add on to the flashing to get extra coverage?