Don’t know what type of vent you have.  I had a Plastimo dorade vent on my coachroof which I removed and replaced with a solar vent.  The hole needed for the solar vent was about 10mm diameter bigger, so I had to enlarge it (taped a carrier bag to the roof on the inside to stop dust going into the boat, and then used a drill with sanding wheels to enlarge the hole.

    The solar vent I used was a Chinese one from ebay, and about 5 or 6 years on, it’s still going strong.  I have it set on extract, and it works well to keep some air circulating through the boat when it is closed up.  The solar fan cost about £40 if I remember right.