On my 30' Hunter, I have two Nicro day and night 3" fans. One in each hatch.
The life is about six to seven years. Shop the internet for the best price which you will not find at West Marine.
If you place in the hatches, tape the area to be cut out, and use a hole saw that is the correct size. Use a clear calk and reverse the inside plate and just drill bolt holes through the hatch. Not a hard job.
Good luck